The document that formally established the United States as an independent nation in 1783 is the Treaty of Paris. This treaty was signed on September 3, 1783, and marked the conclusion of the American Revolutionary War. It recognized the sovereignty of the United States and established its borders, granting the country territory from the Atlantic Ocean to the Mississippi River. The Treaty of Paris was a crucial diplomatic achievement for the United States, as it validated the independence declared in the Declaration of Independence and set the stage for further negotiations and relationships with other nations.