On this day – March 2, 1776 – the so-called Seige of Boston began when under orders from General George Washington – the American artillary forces began the bombardment of Boston. The roots of the seige are actually found back on April 19, 1775 with the battles of Lexington and Concord. Boston was surrounded by the rebel New England militiamen who created an almost total blockade of the town and the British army. The eleven month seige ended when the British commander William Howe, realizing he could no longer hold the town, chose to evacuate it. He withdrew the British forces, departing on March 17 – now celebrated as Evacuation Day – for Halifax, Nova Scotia.
In advance of the Continental Army’s occupation of Dorchester Heights, Massachusetts, General George Washington orders American artillery forces to begin bombarding Boston from their positions at Lechmere Point, northwest of the city center, on this day in 1776.
After two straight days of bombardment, American Brigadier General John Thomas slipped 2,000 troops, cannons and artillery into position just south of Boston at Dorchester Heights. The 56 cannon involved in the move were those taken at Ticonderoga, New York, by Lieutenant Colonel Benedict Arnold and Ethan Allen with his Green Mountain Boys, which had then been transported to Boston by Colonel of Artillery Henry Knox the previous winter.
By March 5, 1776, the Continental Army had artillery troops in position around Boston, including the elevated position at Dorchester Heights, overlooking the city. British General William Howe realized Boston was indefensible to the American positions and decided, on March 7, 1776, to leave the city. Ten days later, on March 17, 1776, the eight-year British occupation of Boston ended when British troops evacuated the city and sailed to the safety of Halifax, Nova Scotia.
Sources include: http://www.history.com/this-day-in-history/the-siege-of-boston