Short answers, pulled from the story.
The Imperial Japanese Army served as the principal ground force of the Empire of Japan from 1871 to 1945. It formed out of domain armies after the Meiji Restoration and was dissolved after Japan's surrender in 1945, with its functions later succeeded by the Ground Self-Defense Force.
The Imperial Japanese Army fought in the First Sino-Japanese War, the Boxer Rebellion, the Russo-Japanese War, World War One, the Second Sino-Japanese War, and World War II. It also carried out the 1874 Taiwan Expedition and the 1931 invasion of Manchuria.
Yamagata Aritomo is described as the father of the Imperial Japanese Army. He carried forward the military reforms first proposed by Ōmura Masujirō, who was assassinated in 1869 before seeing them built.
The Imperial Japanese Army committed widespread war crimes against civilians and prisoners of war during the Second Sino-Japanese War and World War II. These included the Nanjing Massacre, forced prostitution, death marches, biological warfare, and the execution of prisoners, causing tens of millions of deaths.
In December 1941, an Imperial Japanese Army general earned 550 yen a month, about $126.50, while a private first class earned just 9 yen, about $2.07. An American private of the same era earned roughly $50 a month, about 25 times more than a same-ranked Japanese soldier.
Some soldiers kept fighting for decades after Japan's 1945 surrender. Hiroo Onoda surrendered on Lubang Island in the Philippines in March 1974, and Teruo Nakamura surrendered on the Indonesian island of Morotai in December 1974.