Question

What was most surprising about the outbreak of the Chinese Civil War?

Chiang Kai-shek switched sides to launch an attack on the Communist Party.
The Communist Party leaders once had declared their opposition to war.
The Nationalist Party and the Communist Party formerly were aligned.
The Nationalist Party had much to lose and little to gain in a civil war.

Answer 1

The Nationalist Party and the Communist Party formerly were aligned.

They had been allies in the First and Second United Fronts (cooperating against warlords and later against Japan), so it was striking that they turned on each other and resumed full-scale civil war.