The important document ratified in the United States in 1788 is the U.S. Constitution. This foundational legal document established the national framework of governance and the principles of American democracy. Ratified by the necessary nine states, it created a system of checks and balances among three branches of government: the executive, legislative, and judicial. The Constitution has since been amended to address social changes, rights, and liberties, making it a living document that continues to guide the governance of the United States.