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.

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.

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.

In 2023, there were around six live births per 1,000 inhabitants in Japan, down from about 6.3 in the previous year. The total number of live births in the nation amounted to approximately 727,290 in 2023. Japan’s super aging society Directly after the end of WWII, the live birth rate in Japan was over 30 per 1,000 of population. The rate has constantly dropped in the last decades after the second baby boom (between 1971 and 1974). Meanwhile, the life expectancy of the Japanese people has increased, reaching about 87.1 years for women and 81.7 years for men in 2022. Due to the combination of both factors, Japan has developed into one of the most rapidly aging societies in the world. Almost 30 percent of Japan’s population is currently aged 65 years and older, falling into the “super-aged nation” defined by international institutions and organizations.  Decreasing number of marriages In Japan, the number of births outside of marriage is small. The Japanese government, therefore, considers the decreasing number of marriages as the driving factor behind the country’s fertility decline. As of 2023, the number of marriages per 1,000 Japanese citizens was 3.9, less than half compared to that in the early 1970s. The average age of first marriage has also risen for both men and women. This trend can be partially attributed to the increasing number of employed and therefore financially and socially independent women in the past two decades. The employment rate of women in Japan exceeded 50 percent for the first time in history in 2018.

In 2023, the total population in Japan's capital Tokyo Prefecture amounted to approximately **** million inhabitants. Tokyo's population rebounded after it declined for the first time post-WWII in ****.

By August 1945, U.S. and British Commonwealth forces had pushed the Japanese back through Southeast Asia to the Japanese mainland, while Japanese control in China was weakening each day, and the Soviet Union was rapidly approaching from the west. However, despite inevitable defeat looming over the Empire of Japan, its military leaders encouraged its forces to fight to the death, and Japanese resistance grew in ferocity as they were pushed back to the mainland, most notably on the islands of Iwo Jima and Okinawa. As U.S. leadership realized that an amphibious assault on the Japanese mainland would result in heavy and unnecessary casualties on both sides, including large numbers of civilians, an alternative measure was sought. The USSR's advance also put time pressure on the Americans, as defeating Japan quickly would give the U.S. more time to assert control over the region, without having to split control with the Soviets (as was the case in Germany). The bombs are dropped On May 31, 1945, it was suggested that the U.S. demonstrate the power of the atomic bomb to the Japanese, to convince them to surrender - the plan was rejected, however, in the fear that it was not convincing enough, that the Japanese may shoot down the plane, or that the bomb was a dud - instead, a surprise attack was viewed as the most effective course of action. Following the Trinity Test in July 16, the United States achieved the goal of successfully creating and detonating a nuclear weapon. On July 26, the leaders of the U.S., UK, and China all called for the unconditional surrender of Japan, otherwise Japan would face "prompt and utter destruction". Japan did not surrender, and on the morning of August 6, the Enola Gay B-29 bomber dropped an atomic bomb, known as "Little Boy", over the city of Hiroshima. Japan still refused to surrender, and three days later, another bomb, known as "Fat Man", was dropped over Nagasaki. Japan's surrender was announced on August 15, and formalized on September 2, 1945, bringing WWII to a close. Impact The bomb dropped over Hiroshima was a uranium 235 bomb, which detonated at approximately 580 meters (1,900ft) above the ground and had a yield of 16 kilotons (i.e. it had the same blast force as 16,000 tons of TNT). The bomb dropped over Nagasaki was a plutonium 239 bomb, which detonated at approximately 500 meters (1,650ft) and had a yield of 21 kilotons. The temperatures at the epicenter of the explosions reached around 3,000-4,000 degrees Celsius, and thermal radiation was so strong that it caused third degree burns over a kilometer from ground zero. The majority of those within a one km radius of both explosions were killed instantly through the blast force and intense heat, while those further away were more susceptible to injuries by burning, falling debris and glass, and radiation. Additionally, as the fallout from the blast (i.e. radioactive dust and ash sent into the air by the explosion) returned to the ground it was carried by the wind to populations outside of the blast radius, and radiation sickness then took hold over the days and weeks that followed. Due to the nature of the attack, Japan's population distribution at the time, and the long-term effects of radiation poisoning (which may be responsible for illnesses and death several years after exposure), there is no accepted consensus on the total number of deaths due to the atomic bombings at Hiroshima and Nagasaki -the figures given appear to be on the lower end of estimate ranges, where combined estimates generally vary between 110,000 and 210,000 deaths.

In 1800, it is estimated that approximately 9.4 million people lived in the region of modern-day South Korea (and 13.8 million on the entire peninsula). The population of this region would remain fairly constant through much of the 19th century, but would begin to grow gradually starting in the mid-1800s, as the fall of the Joseon dynasty and pressure from the U.S. and Japan would end centuries of Korean isolationism. Following the opening of the country to foreign trade, the Korean peninsula would begin to modernize, and by the start of the 20th century, it would have a population of just over ten million. The Korean peninsula was then annexed by Japan in 1910, whose regime implemented industrialization and modernization policies that saw the population of South Korea rising from just under ten million in 1900, to over fifteen million by the start of the Second World War in 1939.

The Korean War Like most regions, the end of the Second World War coincided with a baby boom, that helped see South Korea's population grow by almost two million between 1945 and 1950. However, this boom would stop suddenly in the early 1950s, due to disruption caused by the Korean War. After WWII, the peninsula was split along the 38th parallel, with governments on both sides claiming to be the legitimate rulers of all Korea. Five years of tensions then culminated in North Korea's invasion of the South in June 1950, in the first major conflict of the Cold War. In September, the UN-backed South then repelled the Soviet- and Chinese-backed Northern army, and the frontlines would then fluctuate on either side of the 38th parallel throughout the next three years. The war came to an end in July, 1953, and had an estimated death toll of three million fatalities. The majority of fatalities were civilians on both sides, although the North suffered a disproportionate amount due to extensive bombing campaigns of the U.S. Unlike North Korea, the South's total population did not fall during the war.

Post-war South Korea Between the war's end and the late 1980s, the South's total population more than doubled. In these decades, South Korea was generally viewed as a nominal democracy under authoritarian and military leadership; it was not until 1988 when South Korea transitioned into a stable democracy, and grew its international presence. Much of South Korea's rapid socio-economic growth in the late 20th century was based on the West German model, and was greatly assisted by Japanese and U.S. investment. Today, South Korea is considered one of the world's wealthiest and most developed nations, ranking highly in terms of GDP, human development and life expectancy; it is home to some of the most valuable brands in the world, such as Samsung and Hyundai; and has a growing international cultural presence in music and cinema. In the past decades, South Korea's population growth has somewhat slowed, however it remains one of the most densely populated countries in the world, with total population of more than 51 million people.

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.

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.

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.

Differences in army sizes and military manpower of nations played a crucial role in the Second World War. While the Allied nations and their colonies or dominions had significantly larger combined populations than the Axis powers, the difference in the number of service personnel at their disposal was much smaller, especially in the war's early phase. Eastern Front With the annexation of Poland, Czechoslovakia, and the Balkans by mid-1941, the Axis forces in the Eastern theater outnumbered the Soviet Union. Following the launch of Operation Barbarossa in June 1941, the Axis forces were then able to push the Soviets as far back as Moscow - when the frontline stabilized, the Soviet Union was able to call upon its larger population to replenish and reinforce its ranks, eventually outnumbering the enemy the following year. Many Soviet women were also drafted in to fight, with notable and decorated examples serving as snipers, pilots, and artillery operators. Western Allies The combined Allied forces in the Pacific and on the Western European fronts (i.e. those mostly led by the UK and U.S.) always outnumbered the Axis powers. This was largely due to the increased mobilization of the United States military after 1941, particularly in the build up to the Invasion of Normandy on June 6, 1944, as well as the redistribution of Germany's forces from the Eastern to Western fronts in response to this, and the withdrawal of Italy from the war in late 1943. Japan did ramp up its conscription efforts in the final years of the war, but it was not enough to offset the huge numerical advantage of the Allies.

Throughout the 19th century, the share of military personnel employed by the United States government was below 0.2 percent of the total population in most years. There were noticeable spikes in enlistments and conscriptions during the American Civil War (1861-65), the First World War (1917-18*), and Second World War (1941-45*), as well as smaller increases during the Mexican-American War (1946-48) and the Spanish-American War (1898), but figures were generally much lower than the post-WWII era.

Following the Second World War, the United States abandoned many of its isolationist positions as it sought to become the world's leading superpower. This involved stationing millions of troops in overseas bases during the Cold War, in strategically important locations such as West Germany, Japan, and Taiwan. Additionally, involvement in conflicts such as the Korean War (1950-1953) and Vietnam War (1964-1973*) kept military employment high, usually between 1-2 percent until the 1970s. Figures remained just below the one percent mark until the 1990s, when the end of the Cold War and the growing influence of technology in conventional warfare saw a decrease in demand for many traditional combat roles. Despite U.S. involvement in a number of overseas conflicts in the 21st century, military personnel represented less than 0.5 percent of the total population in most years between 2000 and 2016.