The key document that established the United States Constitution was the Constitution itself, which was drafted during the Constitutional Convention in Philadelphia in 1787. Following the failure of the Articles of Confederation, the delegates aimed to create a stronger federal government. The final document outlined the framework for government, defining the separation of powers among the legislative, executive, and judicial branches. It was ratified by the states in 1788 and went into effect in 1789. The Constitution also included a Bill of Rights, added in 1791, which guaranteed essential liberties and protections for individuals, shaping the foundation of American democracy.