The Declaration of Independence, and its text, is likely the best known of any primary document of the United States of America and its citizens. When it was written and finally signed on July 4, 1776 it was meant to serve multiple purposes. It declared the formation of a new country, it proclaimed the rights of the then colonists of England to separate from England and it also listed the grievances of the colonists against, at the time, the most powerful country on earth. In addition, the document was meant to inspire, justify the separation from England in the eyes of the world and allies, and to, finally, cause every colonist to rally behind it.
You may also be interested in:
The Federalist Papers
The Constitution of the United States
The Bill of Rights
Congressional Record