In 1938, the year before the outbreak of the Second world War, the countries with the largest populations were China, the Soviet Union, and the United States, although the United Kingdom had the largest overall population when it's colonies, dominions, and metropole are combined. Alongside France, these were the five Allied "Great Powers" that emerged victorious from the Second World War. The Axis Powers in the war were led by Germany and Japan in their respective theaters, and their smaller populations were decisive factors in their defeat. Manpower as a resource In the context of the Second World War, a country or territory's population played a vital role in its ability to wage war on such a large scale. Not only were armies able to call upon their people to fight in the war and replenish their forces, but war economies were also dependent on their workforce being able to meet the agricultural, manufacturing, and logistical demands of the war. For the Axis powers, invasions and the annexation of territories were often motivated by the fact that it granted access to valuable resources that would further their own war effort - millions of people living in occupied territories were then forced to gather these resources, or forcibly transported to work in manufacturing in other Axis territories. Similarly, colonial powers were able to use resources taken from their territories to supply their armies, however this often had devastating consequences for the regions from which food was redirected, contributing to numerous food shortages and famines across Africa, Asia, and Europe. Men from annexed or colonized territories were also used in the armies of the war's Great Powers, and in the Axis armies especially. This meant that soldiers often fought alongside their former-enemies. Aftermath The Second World War was the costliest in human history, resulting in the deaths of between 70 and 85 million people. Due to the turmoil and destruction of the war, accurate records for death tolls generally do not exist, therefore pre-war populations (in combination with other statistics), are used to estimate death tolls. The Soviet Union is believed to have lost the largest amount of people during the war, suffering approximately 24 million fatalities by 1945, followed by China at around 20 million people. The Soviet death toll is equal to approximately 14 percent of its pre-war population - the countries with the highest relative death tolls in the war are found in Eastern Europe, due to the intensity of the conflict and the systematic genocide committed in the region during the war.

The median age of the population in Japan has steadily been increasing since 1950 and is projected to be around 47.7 years old in 2020. As of 2021, the median age of Japan is the second highest in the world, behind the Principality of Monaco. The elderly in Japan An improved quality of life and regular health checks are just two reasons why Japan has one of the highest life expectancies in the world. The life expectancy from birth in Japan improved significantly after World War II, rising 20 years in the decade between 1945 and 1955. As life expectancy continues to increase, Japan expects difficulties caring for the older generation in the future. Shortages in the service sector are already a major concern, with demand for nurses and care workers increasing. Fertility and birth rates The fertility rate among Japan’s population has been around 1.4 children per woman since 2010. Apart from a small baby boom in the early seventies, the crude birth rate of Japan has been declining since 1950 and is expected to be as low as 7.5 births per thousand people in 2020. With falling birth rates and such a large share of its inhabitants reaching their later years, Japan’s total population is expected to continue declining.

Japan Population Census: Male: Age 30 to 34 Years data was reported at 3,684,747.000 Person in 2015. This records a decrease from the previous number of 4,221,011.000 Person for 2010. Japan Population Census: Male: Age 30 to 34 Years data is updated yearly, averaging 3,849,696.500 Person from Dec 1920 (Median) to 2015, with 20 observations. The data reached an all-time high of 5,421,545.000 Person in 1980 and a record low of 1,805,587.000 Person in 1945. Japan Population Census: Male: Age 30 to 34 Years data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G002: Population: Annual.

The 6th Population Census, this is an extraordinary census conducted just after the end of the war. In order to clarify the state of Japan’s population and households, the population census has been conducted in Japan almost every five years.More details on the "Population Census of Japan" overall including other years can be found here: https://d-infra.ier.hit-u.ac.jp/Japanese/statistical-yb/b001.html. 　　　　　Following the enactment of the Statistics Act, the Population Census was implemented as the "Designated Statistics No. 1" based on the Statistics Act. The census examined, for example, whether people had returned to Japan after the war. Due to the war, no Population Census was conducted in 1945.

Japan Population Census: Male: Age 25 to 29 Years data was reported at 3,255,717.000 Person in 2015. This records a decrease from the previous number of 3,691,723.000 Person for 2010. Japan Population Census: Male: Age 25 to 29 Years data is updated yearly, averaging 3,875,527.000 Person from Dec 1920 (Median) to 2015, with 20 observations. The data reached an all-time high of 5,426,289.000 Person in 1975 and a record low of 1,603,664.000 Person in 1945. Japan Population Census: Male: Age 25 to 29 Years data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G002: Population: Annual.

It is estimated that the Second World War was responsible for the deaths of approximately 3.76 percent of the world's population between 1939 and 1945. In 2022, where the world's population reached eight billion, this would be equal to the death of around 300 million people.

The region that experienced the largest loss of life relative to its population was the South Seas Mandate - these were former-German territories given to the Empire of Japan through the Treaty of Versailles following WWI, and they make up much of the present-day countries of the Marshall Islands, Micronesia, the Northern Mariana Islands (U.S. territory), and Palau. Due to the location and strategic importance of these islands, they were used by the Japanese as launching pads for their attacks on Pearl Harbor and in the South Pacific, while they were also taken as part of the Allies' island-hopping strategy in their counteroffensive against Japan. This came at a heavy cost for the local populations, a large share of whom were Japanese settlers who had moved there in the 1920s and 1930s. Exact figures for both pre-war populations and wartime losses fluctuate by source, however civilian losses in these islands were extremely high as the Japanese defenses resorted to more extreme measures in the war's final phase.

Japan Population Census: Male: Age 20 to 24 Years data was reported at 3,046,392.000 Person in 2015. This records a decrease from the previous number of 3,266,240.000 Person for 2010. Japan Population Census: Male: Age 20 to 24 Years data is updated yearly, averaging 3,915,965.500 Person from Dec 1920 (Median) to 2015, with 20 observations. The data reached an all-time high of 5,344,885.000 Person in 1970 and a record low of 2,024,120.000 Person in 1945. Japan Population Census: Male: Age 20 to 24 Years data remains active status in CEIC and is reported by Statistical Bureau. The data is categorized under Global Database’s Japan – Table JP.G002: Population: Annual.

Throughout the Common Era, Japan's population saw relatively steady growth between each century. Failed invasions and distance from Asia's mainland meant that Japan was unaffected by many pandemics, primarily bubonic plague, therefore its development was not drastically impeded in the same way as areas such as China or Europe. Additionally, religious practices meant that hygiene was prioritized much more in Japan than in other regions, and dietary customs saw lower rates of meat consumption and regular boiling of water in meals or tea; both of these factors contributed to lower rates of infection for many parasitic or water-borne diseases. Fewer international conflicts and domestic stability also saw lower mortality in this regard, and Japan was an considered an outlier by Asian standards, as some shifting trends associated with the demographic transition (such as lower child mortality and fertility) began taking place in the 17th century; much earlier time than anywhere else in the world. Yet the most significant changes came in the 20th century, as Japan's advanced healthcare and sanitation systems saw drastic reductions in mortality. Challenges Japan's isolation meant that, when pandemics did arrive, the population had less protection and viruses could have higher mortality rates; smallpox has been cited as the deadliest of these pandemics, although increased international contact in the late 19th century brought new viruses, and population growth slowed. Earlier isolation also meant that crop failure or food shortages could leave large sections of the population vulnerable, and, as mentioned, the Japanese diet contained relatively little meat, therefore there was a higher reliance on crops and vegetables. It is believed that the shortage of arable land and the acidity of the soil due to volcanic activity meant that agriculture was more challenging in Japan than on the Asian mainland. For most of history, paddy fields were the most efficient source of food production in Japan, but the challenging nature of this form of agriculture and changes in employment trends gradually led to an increased reliance in imported crops. Post-Sakoku Japan Distance from the Asian mainland was not the only reason for Japan's isolation; from 1603 to 1853, under the Tokugawa shogunate, international trade was restricted, migration abroad was forbidden, and most foreign interaction was centered around Nagasaki. American neo-imperialism then forced Japan to open trade with the west, and Japan became an imperial power by the early-1900s. Japanese expansion began with a series of military victories against China and Russia at the turn of the century, and the annexation of Taiwan, Korea, and Manchuria by the 1930s, before things escalated further during its invasion of China and the Second World War. Despite its involvement in so many wars, the majority of conflicts involving Japan were overseas, therefore civilian casualties were much lower than those suffered by other Asian countries during this time. After Japan's defeat in 1945, its imperial ambitions were abandoned, it developed strong economic ties with the West, and had the fastest economic growth of any industrial country in the post-WWII period. Today, Japan is one of the most demographically advanced countries in the world, with the highest life expectancy in most years. However, its population has been in a steady decline for over a decade, and low fertility and an over-aged society are considered some of the biggest challenges to Japanese society today.

The fertility rate of a country is the average number of children that women from that country will have throughout their reproductive years. From 1800 until 1865, Japan's fertility rate grew quite gradually, from 4.1 children per woman, to 4.8. From this point the fertility rate drops to 3.6 over the next ten years, as Japan became more industrialized. Towards the end of the nineteenth century, Japan's fertility rate grew again, and reached it's highest recorded point in the early 1920s, where it was 5.4 children per woman. Since this point it has been gradually decreasing until now, although it did experience slight increases after the Second World War, and in the early 1970s. In recent decades Japan's population has aged extensively, and today, Japan has the second oldest population and second highest life expectancy in the world (after Monaco). In contrast to this, Japan has a very low birth rate, and it's fertility rate is expected to fall below 1.4 children per woman in 2020.

In 1800, the population of Indonesia was estimated to be approximately 16 million. The population of the island nation would grow steadily over the course of the 19th century, as the Dutch colonial administration launched several initiatives to modernize the region. After reaching 38 million people in 1900, the population of Indonesia would continue to grow until the 1940’s, when the Japanese occupation of the country would see between four to ten million Indonesians moved away from the island nation to be made to work on Japanese military projects, and in combination with wartime famine, this would result in the death or displacement of up to four million Indonesians by the end of the Japanese occupation in 1945. Despite this, Indonesia's population continued to grow throughout these years.

Following the Second World War, Indonesia claimed its independence from the Netherlands, and achieved this in 1949. In the second half of the 20thcentury, the population would continue to grow exponentially in size through the remainder of the 20th century, although the growth rate would slow somewhat in the 1980s, the result of a decline in fertility rate throughout the country which some studies suggest may be attributed to improved access to birth control and improved mass education. In 2020, Indonesia is estimated to have just over 273.5 million people living within its borders, making it the fourth most populous country in the world (behind the U.S. and above Pakistan).

In Japan, the crude birth rate in 1800 was 29.6 live births per thousand people, meaning that approximately three percent of the population had been born in that year. From 1800 to 1865, Japan's crude birth rate rose gradually to around 34 births per thousand people, before dropping relatively sharply to 25 over the next ten years. This was a time of great social and economic reform in Japan, as the country became increasingly urbanized and industrialized. Japan's crude birth rate reached it's highest recorded point in the early 1920s, where the number was almost 35 births per thousand people, and since then it has been decreasing gradually. There were two times in the twentieth century where Japan's crude birth rate increased, after the Second World War, and during the period of economic prosperity in the 1960s and 70s. Since 1975, Japan's crude birth rate has gradually decreased to it's lowest recorded rate ever, and is expected to be at just 7.5 births per thousand people in 2020, making it the second lowest in the world (behind Monaco).

人口普查：男性：年龄25至 29岁在12-01-2015达3,255,717.000人，相较于12-01-2010的3,691,723.000人有所下降。人口普查：男性：年龄25至 29岁数据按年更新，12-01-1920至12-01-2015期间平均值为3,875,527.000人，共20份观测结果。该数据的历史最高值出现于12-01-1975，达5,426,289.000人，而历史最低值则出现于12-01-1945，为1,603,664.000人。CEIC提供的人口普查：男性：年龄25至 29岁数据处于定期更新的状态，数据来源于総務省 統計局，数据归类于Global Database的日本 – 表 JP.G002：人口：年度。

For most of the Second World War, the Soviet Union had the largest number of active military personnel each year, peaking at over 12 million people in the war's final years. The Soviet Union's ability to draw from its larger population was decisive in its eventual victory over Germany in 1945, whose eastern forces were greatly diminished by 1944 after it had already suffered heavy losses fighting the Soviets and was then forced to fight on the western front.

Each of the Great Powers had varying conscription systems in place that allowed them to draft citizens into the military. In the U.S., over 50 million men aged 18-45 would register for the draft before the war's end, and over 10 million of these were inducted into the military (alongside volunteers and those already in service). Conscription in Japan grew more aggressive as the war progressed, and its armed forces doubled in size in the final two years of the war.

Estimates for the total death count of the Second World War generally range somewhere between 70 and 85 million people. The Soviet Union suffered the highest number of fatalities of any single nation, with estimates mostly falling between 22 and 27 million deaths. China then suffered the second greatest, at around 20 million, although these figures are less certain and often overlap with the Chinese Civil War. Over 80 percent of all deaths were of those from Allied countries, and the majority of these were civilians. In contrast, 15 to 20 percent were among the Axis powers, and the majority of these were military deaths, as shown in the death ratios of Germany and Japan. Civilian deaths and atrocities It is believed that 60 to 67 percent of all deaths were civilian fatalities, largely resulting from war-related famine or disease, and war crimes or atrocities. Systematic genocide, extermination campaigns, and forced labor, particularly by the Germans, Japanese, and Soviets, led to the deaths of millions. In this regard, Nazi activities alone resulted in 17 million deaths, including six million Jews in what is now known as The Holocaust. Not only was the scale of the conflict larger than any that had come before, but the nature of and reasoning behind this loss make the Second World War stand out as one of the most devastating and cruelest conflicts in history. Problems with these statistics Although the war is considered by many to be the defining event of the 20th century, exact figures for death tolls have proven impossible to determine, for a variety of reasons. Countries such as the U.S. have fairly consistent estimates due to preserved military records and comparatively few civilian casualties, although figures still vary by source. For most of Europe, records are less accurate. Border fluctuations and the upheaval of the interwar period mean that pre-war records were already poor or non-existent for many regions. The rapid and chaotic nature of the war then meant that deaths could not be accurately recorded at the time, and mass displacement or forced relocation resulted in the deaths of many civilians outside of their homeland, which makes country-specific figures more difficult to find. Early estimates of the war’s fatalities were also taken at face value and formed the basis of many historical works; these were often very inaccurate, but the validity of the source means that the figures continue to be cited today, despite contrary evidence.

In comparison to Europe, estimate ranges are often greater across Asia, where populations were larger but pre-war data was in short supply. Many of the Asian countries with high death tolls were European colonies, and the actions of authorities in the metropoles, such as the diversion of resources from Asia to Europe, led to millions of deaths through famine and disease. Additionally, over one million African soldiers were drafted into Europe’s armies during the war, yet individual statistics are unavailable for most of these colonies or successor states (notably Algeria and Libya). Thousands of Asian and African military deaths went unrecorded or are included with European or Japanese figures, and there are no reliable figures for deaths of millions from countries across North Africa or East Asia. Additionally, many concentration camp records were destroyed, and such records in Africa and Asia were even sparser than in Europe. While the Second World War is one of the most studied academic topics of the past century, it is unlikely that we will ever have a clear number for the lives lost in the conflict.

Over the course of the Second World War approximately 127.2 million people were mobilized. The world's population in 1940 was roughly 2.3 billion, meaning that between five and six percent of the world was drafted into the military in some capacity. Approximately one in every 25 people mobilized were women, who generally served in an administrative or medical role, although hundreds of thousands of women did see active combat. Largest armies In absolute numbers, the Soviet Union mobilized the largest number of people at just under 34.5 million, and this included roughly 35 percent of the USSR's male population. By the war's end, more Soviets were mobilized than all European Axis powers combined. However, in relative terms, it was Germany who mobilized the largest share of its male population, with approximately 42 percent of men serving. The USSR was forced to find a balance between reinforcing its frontlines and maintaining agricultural and military production to supply its army (in addition to those in annexed territory after 1941), whereas a large share of soldiers taken from the German workforce were replaced by workers drafted or forcibly taken from other countries (including concentration camp prisoners and PoWs). Studying the figures The figures given in these statistics are a very simplified and rounded overview - in reality, there were many nuances in the number of people who were effectively mobilized for each country, their roles, and their status as auxiliary, collaborative, or resistance forces. The British Empire is the only power where distinctions are made between the metropole and its colonies or territories, whereas breakdowns of those who fought in other parts of Asia or Africa remains unclear. Additionally, when comparing this data with total fatalities, it is important to account for the civilian death toll, i.e. those who were not mobilized.

The Second World War had a sever impact on gender ratios across European countries, particularly in the Soviet Union. While the United States had a balanced gender ratio of one man for every woman, in the Soviet Union the ratio was below 5:4 in favor of women, and in Soviet Russia this figure was closer to 4:3.

As young men were disproportionately killed during the war, this had long-term implications for demographic development, where the generation who would have typically started families in the 1940s was severely depleted in many countries.

In 1800, it is estimated that approximately 4.35 million people lived in the region of present-day North Korea. This figure would remain stagnant through much of the 19th century, increasing by just one million people over the next 110 years. The dissolution of the Joseon dynasty in the Korean peninsula in the late-1800s marked the end of centuries of Korean isolationism, and the new Korean Empire then opened itself to foreign trade and influence. By the start of the 20th century, the region of North Korea had a population of just over five million. This growth would increase dramatically following the annexation of the Korean peninsula by Japan in 1910, as extensive industrialization and modernization efforts would result in the population of North Korea doubling from just over five million at the turn of the century, to over ten and a half million by the start of the Korean War in 1950. The Korean War Following Japan's defeat in the Second World War, the Korean Peninsula was split along the 38th parallel, with a Soviet-backed totalitarian government established in the north, and a U.S.-backed authoritarian government in the south. Neither government recognized the legitimacy of the other, and an escalation of tensions saw the North invade the South in June 1950. By 1953, the North's population dropped below ten million. It is estimated that there were approximately three million total fatalities in the war, with North Korea having the largest share of fatalities of all forces involved; however, the North's isolationist and secretive policies have made it difficult to calculate the full impact of the war on the North's population. Most modern estimates suggest that more than 1.5 million North Koreans died or went missing over these three years. Following the establishment of an armistice between North and South Korea in 1953, the population of the newly-formed Democratic People's Republic of North Korea would begin to rise sharply, from 10 million at the time of armistice to almost 15 million in 1970, just twenty years later. North Korea today North Korea's population has increased in each year since the war's end, however, the rate of growth decreased in the late 1970s, as a severe economic crisis and a resulting lack of foreign technology would lead to a scaling back of many of social welfare programs for the country’s citizens. Growth would slow even further following the dissolution of the Soviet Union in 1991, as the loss of Soviet foreign aid would send the North Korean economy into a sharp decline. In the 1990s, a series of flooding, droughts, famine and economic mismanagement led to the deaths of between 500,000 to 600,000* North Koreans. Recent years have seen an improvement in political relations between both the North and South, and even sporadic improvements with the U.S., which could lead to a less-isolated North Korea in the future; however, the continued testing of nuclear weapons and allegations of widespread human rights violations have drawn widespread criticism from the international community. In 2020, it is estimated that approximately 25.8 million people reside in North Korea.

The Second World War was fought on such a large scale that it became total war in many countries - this is where the war effort is prioritized above all else, and the entire population and economy are mobilized to support all military endeavors. Germany and Japan were committing over 70 percent of their national income to the war effort in its final years.

There were also notable fluctuations that coincided with major events for corresponding powers. These included the UK's mobilization of its defenses in 1940, after Germany took most of Western Europe; the spike in Soviet military spending after Operation Barbarossa in June, 1941; and the U.S. entry into the war following the Pearl Harbor attacks in December, 1941.