Complex sentence: Some National Revolutionary Army commanders with Communist backgrounds who were graduates of Whampoa Military Academy kept their sympathies for the Communists hidden and were not arrested, and many switched their allegiance to the CPC after the start of the Chinese Civil War.
Split into simpler sentences: Some National Revolutionary Army commanders with Communist backgrounds who were graduates of Whampoa Military Academy kept their sympathies for the Communists hidden and were not arrested. Later many switched their allegiance to the CPC after the start of the Chinese Civil War.