High Quality Content by WIKIPEDIA articles! The Siege of Boston (April 19, 1775 – March 17, 1776) was the opening phase of the American Revolutionary War, in which New England militiamen—who later became part of the Continental Army—surrounded the town of Boston, Massachusetts, to prevent movement by the British Army garrisoned within. The Americans, led by George Washington, eventually forced the British to withdraw from the town after an 11-month siege.