The stalemate in Boston lasted for months, only breaking when Colonel Henry Knox returned from Fort Ticonderoga in New York, having led a team of sleds loaded with tens of thousands of pounds of artillery (cannon) in winter from the fort across hundreds of miles to Boston. This added artillery gave Washington the firepower needed to make a decisive move. On the night of March 4, 1776, as 800 American soldiers stood guard along the river of Dorchester shores, 1,200 American soldiers occupied Dorchester Heights. They began working through the night to build structures suitable to defend against the British Army. A large portion of the artillery, pulled by oxen, was moved and installed, without being noticed by the British, at Dorchester Heights, a point of strategic importance due to its elevation and commanding view of all of Boston and Boston Harbor.

In response, Howe planned a counteroffensive to take the fortified positions on the Heights, but bad weather forced him to reconsider. The Royal Navy evacuated the British Army from Boston on March 17, 1776, along with many Loyalists.