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.

[2] Arnold, in conjunction with Ethan Allen, his Green Mountain Boys, and militia forces from Connecticut and western Massachusetts, captured the fort and all of its armaments on May 10.

[3] After George Washington took command of the army outside Boston in July 1775, the idea of bringing the cannons from Ticonderoga to the siege was raised by Colonel Henry Knox.

Knox went to Ticonderoga in November 1775, and, over the course of three winter months, moved 60 tons[4] of cannons and other armaments by boat, horse and ox-drawn sledges, and manpower, along poor-quality roads, across two semi-frozen rivers, and through the forests and swamps of the sparsely inhabited Berkshires to the Boston area.

Early in the siege, on June 15, the British agreed on the plan of seizing both of these heights, beginning with those in Dorchester, which had a better view of the harbor than the Charlestown hills.

[11] When Washington took command of the siege in July 1775, he considered taking the unoccupied Dorchester Heights, but rejected the idea, feeling the army was not ready to deal with the likely British attack on the position.

[12] The subject of an attempt on the heights was again discussed in early February 1776, but the local Committee of Safety believed the British troop strength too high, and important military supplies like gunpowder too low, to warrant action at that time.

[17] This cannonade was continued on three successive nights, and while the British were focused on this, the Americans made preparations to implement a plan devised by Rufus Putnam to break the long siege.

Howe and his staff then determined to contest the occupation of the heights, and made plans for an assault, preparing to send 2,400 men under cover of darkness to attack the position.

[29] By the time the storm subsided, Howe reconsidered launching an attack, reasoning that preserving the army for battle elsewhere was of higher value than attempting to hold Boston.

[30] On March 8, intermediaries delivered an unsigned paper [31] informing Washington that the city would not be burned to the ground if Howe's troops were allowed to leave unmolested.

[32] After several days of activity, and several more of bad weather, the British forces departed Boston by sea on March 17 and sailed to Halifax, Nova Scotia, taking with them more than 1,000 Loyalist civilians.