Chinese civil war
What: The war was interrupted when Japan invaded China in 1936 and by World War Two. The war was fought between the nationalist government of China called Kuomintang.
When: 1927 -1936
Why: When the Japanese invaded Japan in 1937 the two sides still again united in order to fight the Japanese this continued throughout World War II. however the two aside still hated each other after the war in 1945 the two sides started up their Civil War with American support. Chiang Kai-shek chair moved his soldiers to the major cities of China however the CPC was heavily found funded by the soviet union and quickly gained support in the rural areas.
How: Because they do not like each other.