Complex sentence: Unbeknown to the nationalists, the communists was setting up a trap by giving up more land in order to make the nationalist spread thin, and the primary objective of the communists were to annihilate the nationalist 21st Division and the 43rd Division.
Split into simpler sentences: Unbeknown to the nationalists, the communists set a trap by giving up more land to spread the nationalist thin. The primary objective of the communists was to annihilate the nationalist 21st Division and the 43rd Division.