Ever since he was given command of the Continental Army by the Second Continental Congress, General George Washington wanted to take Boston back from the British. The skirmishes at Lexington and Concord were not fought by an organized army, but by the local militiamen. Because the delegates to the Second Continental Congress thought the war would not last long, men only enlisted for a year.
In January 1776 when Thomas Paine was writing his Common Sense, George Washington was camped outside Boston begging his men to stay to the end of December when more recruits would arrive. Some stayed, but many left. At that time, the Continental Army was low on men and low on armory to fight any battle. With the loss at Bunker (Breed's) Hill, a successful attack on Boston didn't seem possible.
When the Continentals captured Fort Ticonderoga and Crown Point on Lake Champlain, they also captured cannons and mortars. These would useful in their fight to take Boston. General Washington sent Colonel Henry Knox to the north to bring them back to Boston.
Colonel Knox arrived with 80 sleds packed with cannons, mortars and other heavy equipment in February. General Washington saw his chance. Even though 2,000 of his 9,000 soldiers didn't have muskets, he worked out the perfect plan. On March 2 and 3, 1776, soldiers fired all night into the city of Boston from the west. This was a camouflage of what was really happening.
There were hills south of Boston named Dorchester Heights which reminded General Washington of Bunker and Breed's Hills. He knew they would be perfect places to set up his heavy artillery. While the armed soldiers were firing into the city of Boston at night, the men who didn't have guns moved the artillery to Dorchester Heights. General Washington had them place the cannons pointed towards the British camps in the city.
On the morning of March 4th General Howe noticed how many cannons were pointed at him and his troops. He couldn't believe how much work the Continentals had done in just one night. "The rebels have done more in one night than my whole army could do in months." He didn't realize it took two nights, because the Continentals were careful to hide the evidence of their first night's work.
General Howe knew that Washington's troops had placed their artillery on Dorchester Heights so they could command Boston, threaten the British Army, and make Boston Harbor unsafe for any British ship. First General Howe planned to attack the Continentals on Dorchester Heights. In the end, he decided just to leave Boston and move his troops to New York. New York was more important because whoever controlled it would also be able to control the traffic to and from Canada.
General Howe and his troops with 1,000 Loyalists sailed for New York. On March 17, 1776, the Continental Army led by General George Washington marched back into Boston. At this time, there were no British troops in any of the thirteen colonies.