The Siege of Boston (April 19, 1775 – March 17, 1776) is the opening phase of the American Revolutionary War.

New England militiamen prevent  the movement by land of the British Army garrisoned in what is at this time the peninsular city of Boston, Massachusetts.

Both sides have to deal with resource supply and personnel issues over the course of the siege.

British resupply and reinforcement activities are limited to sea access.

After eleven months of the siege, the British abandon Boston by sailing to Nova Scotia.

The siege begins on April 19 after the Battles of Lexington and Concord, when the militia from surrounding Massachusetts communities block  land access to Boston.

The Continental Congress forms the Continental Army from the militia, with George Washington as its Commander in Chief.

In June 1775, the British seizes Bunker and Breed's Hills, from which the Continentals are preparing to bombard the city, but their casualties are heavy and their gains are insufficient to break the Continental Army's hold on land access to Boston.

The Americans lay siege to the British-occupied city.

Military actions during the remainder of the siege are limited to occasional raids, minor skirmishes, and sniper fire.

In November 1775, Washington sends the twenty-five-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 brings many cannons to the Boston area by January 1776.

In March 1776, these artillery fortify Dorchester Heights (which overlooks Boston and its harbor), thereby threatening the British supply lifeline.

The British commander William Howe sees the British position as indefensible and withdraws the British forces in Boston to the British stronghold at Halifax, Nova Scotia, on March 17 (celebrated today as Evacuation Day).