Claim: George Washington was commander-in-chief of the Continental Army.

The Siege of Boston -LRB- April 19 , 1775 -- March 17 , 1776 -RRB- was the opening phase of the American Revolutionary War . New England militiamen prevented the movement by land of the British Army garrisoned in what was then the peninsular city of Boston , Massachusetts . Both sides had to deal with resource supply and personnel issues over the course of the siege . British resupply and reinforcement activities were limited to sea access . After eleven months of the siege , the British abandoned Boston by sailing to Nova Scotia .   The siege began on April 19 after the Battles of Lexington and Concord , when the militia from surrounding Massachusetts communities blocked land access to Boston . The Continental Congress formed the Continental Army from the militia , with George Washington as its Commander in Chief . In June 1775 , the British seized Bunker and Breed 's Hills , from which the Continentals were preparing to bombard the city , but their casualties were heavy and their gains were insufficient to break the Continental Army 's hold on land access to Boston . Military actions during the remainder of the siege were limited to occasional raids , minor skirmishes , and sniper fire .   In November 1775 , Washington sent the 25-year-old bookseller-turned-soldier Henry Knox to bring to Boston the heavy artillery that had been captured at Fort Ticonderoga . In a technically complex and demanding operation , Knox brought many cannons to the Boston area by January 1776 . In March 1776 , these artillery fortified Dorchester Heights -LRB- which overlooked Boston and its harbor -RRB- , thereby threatening the British supply lifeline . The British commander William Howe saw the British position as indefensible and withdrew the British forces in Boston to the British stronghold at Halifax , Nova Scotia , on March 17 -LRB- celebrated today as Evacuation Day -RRB- .