On the night of 4 March 1776, Continental Army batteries positioned at Cambridge, just east of Boston, opened fire on the British-occupied city. It seemed a waste of precious ammunition, because the barrage had little effect other than to cause the British to expend some of their more abundant ammunition to retaliate. But what the British did not know was that while they were preoccupied with returning fire, 2,000 American troops ascended Dorchester Heights south of the city and began digging in. Using hay bales positioned to muffle the sounds, the Americans hauled up the slopes a number of heavy cannon that they had captured from Fort Ticonderoga in New York.
At dawn, the British were astonished to see the maws of American guns pointed at them from the tactically advantageous heights to their south. It was a brilliant American tactical maneuver. With the memory of the Battle of Bunker Hill still fresh in his mind, British General William Howe rejected the idea of another costly assault on higher ground and decided instead to evacuate Boston.