The 11-month siege of Boston ended when the Continental Army, under the command of George Washington, fortified Dorchester Heights in early March 1776 with cannons captured at Ticonderoga.
British General William Howe, whose garrison and navy were threatened by these positions, was forced to decide between attack and retreat. To prevent what could have been a repeat of the Battle of Bunker Hill, Howe decided to retreat, withdrawing from Boston to Nova Scotia on March 17.
The British evacuation was Washington's first victory of the war. It was also a huge morale boost for the Thirteen Colonies, as the city where the rebellion began was the first to be liberated.
On February 16, General Washington and his officers held a war council. They decided that they had to take some kind of action before British reinforcements arrived in the spring. They decided to occupy Dorchester Heights, which overlooked Boston Harbor. Since Henry Knox had succeeded in transporting cannon and artillery from the captured Fort Ticonderoga, the Americans were able to lay down fire while fortifications were built on Dorchester Heights.
The operations began on March 2 and by March 5, fortifications were visible to the British. Their cannon could not fire on the elevated position. An assault by the British that evening was called off as a storm moved in. By March 7, General Howe realized that Boston and its harbor were now indefensible in the face of artillery from Dorchester Heights and he decided to evacuate. On March 17, 1776, the British boarded their ships and evacuated the city. On March 27, they sailed to Halifax, Nova Scotia.
The Americans had no idea where the British were heading, but many including General Washington assumed that New York City was their destination. By April 1776, he had moved his headquarters to outside that city and had circulated a warning throughout the city about the possibility of a British invasion. The British would come, but not until August 1776.