Japan began building its military in the 1850s after opening trade with the US. It gained territory after fighting with allies in WWI. Growing nationalism and economic struggles led the military to gain influence in the 1930s. The Japanese army seized Manchuria from China in 1931 in an event known as the "Manchurian Incident" and established Manchukuo as a puppet state under its control. Japan then invaded more of China in 1937, committing atrocities like the Rape of Nanjing. Seeking resources, Japan declared the Greater East Asia Co-Prosperity Sphere and began expanding its empire across Asia.