The key document that established the principles of the new United States government was the Constitution of the United States, ratified in 1788. This foundational document outlines the structure of the government, including the separation of powers among the legislative, executive, and judicial branches. The Constitution also enshrines essential rights and liberties for citizens through the Bill of Rights, which encompasses the first ten amendments. By creating a framework for representative democracy and federalism, it has profoundly influenced the development of political systems around the world, laying the groundwork for modern governance and civil rights.