The Fortification of Dorchester Heights was a decisive action early in the American Revolutionary War that precipitated the end of the siege of Boston and the withdrawal of British troops from that city.
On March 4, 1776, troops from the Continental Army under George Washington's command occupied Dorchester Heights, a series of low hills with a commanding view of Boston and its harbor, and mounted powerful cannons there. General William Howe, commander of the British forces occupying the city, considered contesting this act, as the cannon threatened the town and the military ships in the harbor. After a snowstorm prevented execution of his plans, Howe decided instead to withdraw from the city. The British forces, accompanied by Loyalists who had fled to the city during the siege, left the city on March 17 and sailed to Halifax, Nova Scotia.
The standoff continued throughout the fall and winter. In early March 1776, heavy cannons that the patriots had captured at Fort Ticonderoga were brought to Boston by Colonel Henry Knox, and placed on Dorchester Heights. Since the artillery now overlooked the British positions, Howe's situation was untenable, and the British fled on March 17, 1776, sailing to their naval base at Halifax, Nova Scotia. Washington then moved most of the Continental Army to fortify New York City.