Why did Mao and the Communists take the Long March?

Because they were faced with the prospect of annihilation.

powers  are  denied  to the  national governmentin three distinct ways: some  powers, such as the  power  to levy duties on exports or prohibit the freedom of religion, speech, press, or assembly, are expressly  denied  to the  national government  in the  constitution.

these civilizations were located around fertile rivers, where they could produce an abundance of crops