Key figures: George Washington, William Howe, Henry Knox, John Thomas, Rufus Putnam
Summary
The Fortification of Dorchester Heights was the decisive maneuver that ended the nearly year-long Siege of Boston in March 1776. Dorchester Heights was a pair of low hills on a peninsula south of Boston that commanded both the town and the anchorage of the British fleet in the harbor. Neither side had occupied the position during the siege, but General George Washington recognized that artillery mounted there would make the British position untenable.
The operation was made possible by the arrival of heavy guns from Fort Ticonderoga. Colonel Henry Knox had proposed retrieving the fort’s captured cannon, and over the winter of 1775–1776 he led the “noble train of artillery” — some 59 pieces weighing roughly 60 tons — by boat, ox-drawn sledge, and horse across frozen rivers and the Berkshire hills. The expedition ran from November 1775 until late January 1776, when the train reached Washington’s camp at Cambridge.
To mask the operation, Washington opened a diversionary bombardment of Boston on the nights of March 2 and 3, intensifying it on March 4. That same night, a working party of roughly 2,500 troops under General John Thomas moved onto the heights and, because the frozen ground could not be dug, raised fortifications from prefabricated timber frames (chandeliers), fascines, and earth-filled barrels devised by engineer Rufus Putnam. By dawn on March 5, 1776 — the sixth anniversary of the Boston Massacre — the British awoke to find a fortified position bristling with cannon overlooking their army and ships. British commander General William Howe reportedly remarked that the rebels had done more in one night than his army could have done in a month.
Howe first prepared an assault of some 2,400 men against the heights, but a severe snowstorm on the night of March 5 prevented the attack. Judging the position too strong to storm and his fleet exposed, Howe chose to abandon Boston. On March 17, 1776 — later commemorated as Evacuation Day — British forces and more than a thousand Loyalist civilians embarked and sailed for Halifax, Nova Scotia, ending the siege without a general engagement.
Significance
The fortification of Dorchester Heights delivered the first major strategic victory of the Revolutionary War and drove British forces from a major American city without a pitched battle. It vindicated Washington’s cautious command and demonstrated the value of Knox’s artillery, which would serve the Continental Army through the rest of the war. The bloodless recapture of Boston boosted Patriot morale and confirmed the young army’s ability to compel a professional adversary to retreat through position and maneuver. Strategically, however, the British withdrawal to Halifax was a regrouping rather than a defeat: Howe’s army returned within months to open the New York campaign, and the war’s center of gravity shifted southward to New York and New Jersey for the remainder of 1776.