The Second Sino-Japanese War (1937–1945) became part of World War II (1939–1945). China suffered huge human and material losses during the war. The country was divided into three regions during most of the struggle: a Nationalist-controlled area, a Communist-controlled area, and the regions under Japanese occupation.
The Japanese defeated often stubborn Chinese resistance during the early stages of the war (1937–1938). The Nanjing Massacre of Chinese civilians took place during this period. The Soviet Union, an old enemy of Japan, provided assistance to China.
From 1939–1943, the war was stagnated. The Communists and Nationalists, supposedly united against the Japanese, sometimes fought each other. Inflation, hoarding, and corruption affected areas under Nationalist control. The Nationalists became much weaker. The Communists, on the other hand, gained strength during this phase of the war. The Communists waged guerrilla warfare and groomed their future leaders. They were more effective in fighting Japan, and many patriotic Chinese rallied to their cause.
Although the US provided considerable assistance to China, the situation in China was dire in 1944. Japan occupied more territory, and dissatisfaction with the corrupt Nationalists became more widespread.
After Japan surrendered in 1945, civil war broke out. The ineffective Nationalists were routed. All of China was united by the Communists in 1949.