PERIOD: 1920-1939. NOTE: (As of October 1st but as of September 1st in 1923)The population estimates were obtained as follows: (1) For 1921 to 1923, the population estimate is the sum of county- and city-level population estimates obtained by multiplying the de facto population in the Population Census conducted on October 1, 1920, with the average annual population growth rate by gender from 1908 to 1918. (2) For 1924, the difference between the population of Japan overall calculated using the population growth rate by sex in each city and summing up the results and the population overall calculated using the population growth rate by sex for Japan overall was proportionally subtracted from the population of each prefecture; moreover, the population decrease due to the Great Kanto Earthquake on September 1, 1923 was also taken into account. (3) For Taisho 1926 to 1929, the de facto population in the 1920 and 1925 Population Censuses is used to obtain the annual average geometric growth rate of Japan's population overall, which is then used to estimate the population. (4) For 1931 to 1934, the same procedure is employed using the de facto population in the 1920 and 1930 Population Censuses. (5) From 1926 onward, the population estimates are obtained by adding the increase in the difference between births and deaths up to each estimation year in the 1935 Population Census using the results of the Vital Statistics survey. SOURCE: [Survey by the Statistics Bureau, Imperial Cabinet].

In 1800, the population of Japan was just over 30 million, a figure which would grow by just two million in the first half of the 19th century. However, with the fall of the Tokugawa shogunate and the restoration of the emperor in the Meiji Restoration of 1868, Japan would begin transforming from an isolated feudal island, to a modernized empire built on Western models. The Meiji period would see a rapid rise in the population of Japan, as industrialization and advancements in healthcare lead to a significant reduction in child mortality rates, while the creation overseas colonies would lead to a strong economic boom. However, this growth would slow beginning in 1937, as Japan entered a prolonged war with the Republic of China, which later grew into a major theater of the Second World War. The war was eventually brought to Japan's home front, with the escalation of Allied air raids on Japanese urban centers from 1944 onwards (Tokyo was the most-bombed city of the Second World War). By the war's end in 1945 and the subsequent occupation of the island by the Allied military, Japan had suffered over two and a half million military fatalities, and over one million civilian deaths.

The population figures of Japan were quick to recover, as the post-war “economic miracle” would see an unprecedented expansion of the Japanese economy, and would lead to the country becoming one of the first fully industrialized nations in East Asia. As living standards rose, the population of Japan would increase from 77 million in 1945, to over 127 million by the end of the century. However, growth would begin to slow in the late 1980s, as birth rates and migration rates fell, and Japan eventually grew to have one of the oldest populations in the world. The population would peak in 2008 at just over 128 million, but has consistently fallen each year since then, as the fertility rate of the country remains below replacement level (despite government initiatives to counter this) and the country's immigrant population remains relatively stable. The population of Japan is expected to continue its decline in the coming years, and in 2020, it is estimated that approximately 126 million people inhabit the island country.

PERIOD: 1920, 1925, 1930, 1935 (every Oct. 1). SOURCE: [Survey by the Statistics Bureau, Imperial Cabinet; Statistics by government offices, overseas territories of Japan].

PERIOD: Korea, Taiwan, South Sakhalin and Kwantung Province, 1929-1938 year-end. South Pacific Mandate, every October 1 from 1929 to 1937 and end of June 1938 . NOTE: Excluding local troops each places. Locals in Taiwan include indigenous peoples who have not advanced into Sinicization, and Japanese include Koreans. Japanese of Sakhalin and South Pacific Mandate include Koreans and Taiwanese respectively. Locals in Kwantung Province are Manchurian, and Japanese include Koreans and Taiwanese. Excluding South Manchuria Railway Zone from 1937. SOURCE: [Survey by the Statistics Bureau, Imperial Cabinet; Statistics by government offices, overseas territories of Japan].

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.

During the Second World War, the three Axis powers of Germany, Italy, and Finland mobilized the largest share of their male population. For the Allies, the Soviet Union mobilized the largest share of men, as well as the largest total army of any country, but it was restricted in its ability to mobilize more due to the impact this would have on its economy. Other notable statistics come from the British Empire, where a larger share of men were drafted from Dominions than from the metropole, and there is also a discrepancy between the share of the black and white populations from South Africa.

However, it should be noted that there were many external factors from the war that influenced these figures. For example, gender ratios among the adult populations of many European countries was already skewed due to previous conflicts of the 20th century (namely WWI and the Russian Revolution), whereas the share of the male population eligible to fight in many Asian and African countries was lower than more demographically developed societies, as high child mortality rates meant that the average age of the population was much lower.

Of the 11 present-day countries in Southeast Asia, only Thailand was independent at the outbreak of the Second World War, while the other 10 were in the hands of Western powers. This was one of several sources of Japan's resentment for the West prior to the Second World War, as Western industrial growth had been fueled by resources taken from Asia for centuries. The German invasion of France and the Netherlands in May 1940 greatly weakened the Western position in Southeast Asia, and Japan sought to capitalize on the turmoil in Europe and annex the region into its own empire. Japanese expansion Japan's expansion into Southeast Asia began with its invasion of French Indochina in September 1940, which was used to create a blockade China, with whom Japan was also at war. However, the United States' Pacific fleet remained a major threat in the area, and the U.S. was growing wary of Japan's ambitions (but had not yet become hostile). Because of this, Japan launched a surprise attack on the American naval base at Pearl Harbor, Hawaii, on December 7, 1941, to try and hinder the American response to its planned aggression in the Pacific. The attack was a success, and Japanese forces then began attacking and moving into other areas of Southeast Asia later that day (December 8th, local time), and had taken virtually all of the 11 countries within six months. The Dutch East Indies (Indonesia) was one of the most important assets taken by the Japanese, due to its abundant resources, especially fossil fuels. Most Indonesians welcomed the Japanese as liberators, but animosity quickly grew as millions were then put to work as forced laborers across Japan's territories, and up to four million Indonesian civilians would die of famine, abuse, or forced labor by the war's end. End of WWI and independence The Japanese advance stalled in New Guinea in mid-1942, where the joint forces of Australia and New Zealand were able to gain a foothold on the island. The United States victory in the Battle of Midway also dealt a heavy blow to the Japanese Navy in the Pacific, proving to be a turning point of the Pacific War. In the years that followed, the Allies gradually pushed the Japanese northwards through Southeast Asia, before Japan's eventual defeat in 1945. The post-WWII years in Southeast Asia were defined by the push for independence and decolonization - between 1945 and 1957, nine of the ten colonized countries would gain their independence (Timor-Leste remained Portuguese until 1975, when it was then invaded and annexed by Indonesia from 1976-2002).

PERIOD: Total for the Empire of Japan, FY1936. Japan proper, FY 1932-1936. Korea, Taiwan, South Sakhalin, Kwantung Province and South Pacific Mandate, FY 1934-1936. By school for FY1936. SOURCE: [Survey by the Ministry of Education; Statistics by government offices, overseas territories of Japan].

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.