Warmatrix

18th infantry regiment during Battle of Dachang

As the name implies, the Japanese Imperial Army was the army of the Japanese empire. It existed from 1871 CE to 1945 CE.
Before 1868 CE, Japan was a feudal shogunate, almost completely sealed off from the world. When the Americans forced the Japanese government to open its ports to trade, the Meji Restoration transformed the nation. The emperor, until then a puppet ruler, was restored to power and the country modernized at breakneck speed. In 1871 CE the feudal han system was abolished; two years later conscription was adopted. The new army was modeled on European examples. Disgruntled samurai, deprived of their autocracy, launched several revolts, most notably in the Satsuma rebellion, but were quickly defeated.
Besides internal dissent, Japan also felt threatened by external western powers. To prevent itself from being overwhelmed, it set out to become a world power itself. On top of an industrial base a modern army and navy were created, which repeatedly competed with each other for power and overall strategy. Ironically their organization was improved by advisers from European states. Throughout its lifetime the army grew steadily: a humble 12,000 men at its foundation; 36,000 a few years later; 250,000 - 400,000 throughout the early 1900's CE; nearly 500,000 men in 1941 CE; 5 million at the end of World War II.
Around 1890 CE the Japanese Imperial Army was the most modern army in Asia. The Japanese government intended to use it in expanding its influence in Korea, China and further south, aiming to create a buffer between itself and Russia and also gaining land and resources in China. The army saw its first major action in the First Sino-Japanese War and during the Boxer Rebellion, where it defeated the ruling Chinese Qing dynasty and rebels respectively. Its real test was the Russo-Japanese War of 1904 CE - 1905 CE, where, together with the navy, it defeated Russia, surprising both friend and foe.
In World War I Japan fought briefly on the side of the Entente, using the conflict only to take Qingdao from Germany. After the war, it continued to expand its influence in China. Western powers were wary of Japan's ambitions and imposed measures to contain the country. In the 1926 CE, after an internal power struggle, right wing forces with close ties to the military came to power, starting the Showa period. The new government set out on a nationalistic and imperialistic course. The army was expanded and renamed from "rikugun" (army) to "kogun" (imperial army). All soldiers were required to swear loyalty to the emperor. They were trained harshly and instilled with a simplified form of "busihdo", the warrior code of the samurai, which was mainly condensed into the ideal of "gyokusai" (glorious death) before dishonor. As a result, discipline in battle was superb, up to the level of fanaticism. However it discouraged initiative and independent thinking among the officers. This led to grave tactical mistakes in later conflicts.
When the Great Depression hit the country, control slipped from the civilian government. In 1931 CE the army, acting on its own, provoked the Mukden Incident and then invaded Manchuria. There were several bouts with the Soviet Union and China. In 1936 CE the government surrendered its powers almost completely to the military. A year later the Second Sino-Japanese War broke out. During that war, the army reinforced its reputation of fanaticism and brutality. Their training taught the soldiers not only discipline, but also contempt for the fate of prisoners. The Japanese regarded enemy soldiers who surrendered as dishonorable and mistreated them badly, in many cases using them as a labor force and/or wounding and starving them. Civilians too, considered racially inferior, were treated brutally. The most extreme example was the Nanking Massacre of 1937 CE - 1938 CE, where somewhere between 40,000 and 300,000 people were killed.
When the World War II broke out in 1939 CE, Japan was still heavily occupied in China. Relations with the USA were already sour and when the Americans imposed heavy trade embargoes, Japan decided on war. The strategy was to knock out the American Pacific fleet and establish a defensive ring of islands before the USA could rebuild its navy. A surprise attack was launched on Pearl Harbor and the army quickly overran most of east Asia. The army was well suited for that task. It used motorized vehicles, horses and bicycles to achieve high mobility. Supported by good artillery and the air force it overcame less modern Asian armies in lightning campaigns.
But the USA recovered quickly from the first attack and struck back, forcing the imperial army to fight defensively. In the war its light tanks, inferior firearms and unarmored airplanes proved very vulnerable to counterattack. The soldiers suffered heavily from tropical diseases because the Japanese medical corps was underdeveloped and had shortages of medicine. Yet they fought on, tenaciously defending a ring that steadily closed in on the homeland of Japan. Only economic strangulation, the Soviet Manchurian campaign, strategic bombing and finally the atomic bombs forced Japan to surrender.
After the war, the Imperial Army was disbanded and replaced by the Ground Self Defense Force, a smaller army, by law committed to defense only. A handful of old soldiers refused to give up the fight; the last ones surrendered in 1974 CE.