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.
The siege began on April 19 after the Battles of Lexington and Concord, when the militia from many Massachusetts communities surrounded Boston and blocked land access to the then-peninsular town, limiting British resupply to naval operations. The Continental Congress chose to adopt the militia and form the Continental Army, and unanimously elected George Washington as its Commander in Chief. In June 1775, the British seized Bunker and Breeds Hills, but the casualties they suffered were so heavy and their gains were insufficient to break the siege. For the rest of the siege, there was little action other than occasional raids, minor skirmishes, and sniper fire. Both sides had to deal with resource supply and personnel issues over the course of the siege, and engaged in naval operations in the contest for resources.
In November 1775, Washington sent a 25 year-old bookseller-turned-soldier named Henry Knox to bring heavy artillery that had been captured at Fort Ticonderoga to Boston. In a technically complex and demanding operation, Knox brought many cannons to the Boston area in January 1776. In March 1776, these artillery were used to fortify Dorchester Heights, overlooking Boston and its harbor and threatening the British naval supply lifeline. The British commander William Howe, realizing he could no longer hold the town, chose to evacuate it. He withdrew the British forces, departing on March 17 (celebrated today as Evacuation Day) for Halifax, Nova Scotia.