The Chinese Nationalist Party and the the Communist Party of China came together to form an alliance during the Second Sino-Japanese War, when Japan tried to invade China.
Step-by-step explanation:
Before the happening of the Second Sino-Japanese War in 1937, the Chinese Civil War was already going on, fought between the Chinese Nationalist Party and the Communist Party of China. However, when Japan tried to invade China in 1937, the then Chinese leader Chiang Kai-shek realized that China was ill-prepared for a full fledged war against China, given the chaotic domestic situation. As a result, he decided to unite China against Japan and thus the Nationalists and the Communists came together to form the alliance which was referred to as the "Second United Front".