The United States Constitution reached the required nine-state ratification threshold on June 21, 1788.
New Hampshire became the ninth state to ratify the document on that date. Article Seven specified that ratification by conventions in nine states would establish the Constitution among the states approving it, replacing the Articles of Confederation as the basis for the new federal government.
The Constitution had been signed in Philadelphia on September 17, 1787, but signing did not put it into operation. Ratification debates divided Federalists, who supported the new framework, and Anti-Federalists, who worried about centralized power and the lack of a bill of rights.
North Carolina and Rhode Island ratified later, after the nine-state threshold had been reached. The federal government began operating under the Constitution on March 4, 1789, so the ratification date and the government’s first operating date are separate milestones.