The U.S. Constitution went into effect on March 4, 1789, by agreement of the Confederation Congress. It was written during the Constitutional Convention, held from May to September in 1787, and it was signed on Sept. 17, 1787.
The delay from the signing of the Constitution to its enactment occurred because it had to be ratified, a process that allowed the people to decide whether it represented their interests. The Constitution created the foundation for the country’s system of government. It gave power to the people by separating the government into three branches, dividing power between the state and federal governments, and it established checks and balances to prevent too much power from going to any one person or branch.