The correct sequence of events that led to the Communist Revolution in China is option A:
A. Communists help nationalists secure the cities → nationalists overthrow dynastic rule → nationalists purge the communists → communists and nationalists fight the Japanese and each other → communists overtake the nationalists after WWII.
This sequence accurately reflects the historical context where the Communists initially worked with the Nationalists against the warlords and the imperialist forces, but later conflicts arose leading to the purge of Communists by the Nationalists. During World War II, both factions fought against Japanese invaders, but post-war, the struggle resumed, and the Communists eventually gained the upper hand leading to their victory in 1949.