Thirteen colonies joined together in the Declaration of Independence adopted in 1776.
The thirteen British colonies along the Atlantic coast declared themselves free and independent states through the actions of the Second Continental Congress. They were New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, Virginia, North Carolina, South Carolina, and Georgia.
The colonies did not all have identical histories or political views. Georgia had not attended the First Continental Congress in 1774, but it sent delegates to the Second Continental Congress and joined the independence movement. The Declaration’s opening refers to the “thirteen united States of America.”
The phrase “thirteen colonies” refers specifically to the British colonies that became the original states. The United States later expanded through admission of new states and the creation of territories, so the number at the founding should not be confused with the modern number of states.