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, 1776.