Why did some colonists form militias?

They did this because the British were sending soldiers into Boston and they had no way to defend them. Thus, setting up militias.

First, the militia served in place of a standing army to resist foreign aggression. Second, the militia served as an internal police force for the states. Third, following the establishment of the federal government, the militia served to resist or deter the use of a federal standing army against the states.

Explanation:

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