The document that outlined the principles of governance for the United States is the Constitution, adopted in 1787. It established the framework for the federal government and delineated the separation of powers among the three branches: legislative, executive, and judicial. The Constitution also set forth the Bill of Rights, ensuring individual liberties such as freedom of speech, religion, and the right to a fair trial. This foundational document has provided stability and continuity to American governance, becoming a model for other democratic nations and inspiring movements for civil rights and social justice throughout history.