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Immediately after Lexington and Concord, Boston was invested by 16,000 Americans under General Ward. The first attempt to narrow the lines resulted in the defeat of the Americans at Bunker Hill (q. v.). On July 3 Washington succeeded Ward. Through the fall and winter he drilled his men and drew his siege lines closer. He waited only for ammunition and siege guns to begin more active operations. Having secured these in the early spring of 1776 from Ticonderoga (q. v.), he seized Dorchester Heights on the night of March 4, 1776. Here he commanded the city and the fleet, and on March 17 Howe, who had succeeded Gage, was obliged to evacuate Boston.