The main purpose of the United States Declaration of Independence is to...

guarantee human rights for all Americans.
explain and justify why the American colonists revolted against their mother country.
provide a clear plan for a meaningful and effective political system.
establish the basic law of the land in the United States.

The main purpose of the United States Declaration of Independence is to explain and justify why the American colonists revolted against their mother country. It outlines the grievances that the colonists had against British rule and asserts their right to freedom and self-governance. The Declaration of Independence served as the catalyst for the American Revolution and ultimately led to the establishment of the United States as an independent nation.