The Siege of Boston (April 19, 1775 – March 17, 1776) was the opening phase of the American Revolutionary War, in which New England militiamen—and then the Continental Army—surrounded the city of Boston, Massachusetts, to prevent movement by the British Army garrisoned within. As a siege it was only partially successful, but it played an important role in the creation of the Continental Army and promoting the unity of the Thirteen Colonies. It also served to shape the attitudes and character of participants on both sides. The most important single event of the siege was the Battle of Bunker Hill.