American Revolutionary War. Siege of Boston (April 19, 1775 - March 17, 1776). Departure of the British fleet. On March 17, 1776, British forces were forced to evacuate the city of Boston following General George Washingtons successful placement of fortifications and artillery on Dorchester Heights, which overlooks Boston fom the south. Engraving by Petit. Panorama Universal. History of the United States of America, from 1st edition of Jean B.G. Roux de Rochelle's Etats-Unis d'Amerique in 1837. Spanish edition, printed in Barcelona, 1850.