On June 17, 1775, the British suffered the embarrassment of the Battle of Bunker Hill. They won the battle and took the hill, but only after three assaults and absorbing tremendous casualties. In July, newly appointed Continental Commander-in-Chief General George Washington arrived and took command of the siege. He immediately began training the militia into a regular army. In September, General Gage was officially recalled to England. He turned command over to General Howe on October 10, 1775.
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.