In 1773, the Sons of Liberty dumped tea into Boston harbor in protest of the Tea Act, in what became known as the Boston Tea Party. In response, Gage was appointed the military governor of Massachusetts in 1774. His troops occupied Boston, and the Coercive Acts were passed by the British Parliament. These laws replaced the elected Massachusetts legislature with an appointed one and closed Boston harbor in an attempt to suppress the rebellious acts against British authorities.
As a result of the acts, relations merely tensed, and by April 1775, colonial militias had begun gathering arms and ammunition in preparation for potential hostilities. When Gage learned of this, he sent British troops to disarm the rebels and arrest the rebels Samuel Adams and John Hancock in the area around Lexington and Concord. He chose Marine Major John Pitcairn for the task, knowing of the man's reputation as a negotiator.
The rebel Joseph Warren had learned of Gage's plans to arrest Adams and Hancock, and sent Paul Revere and William Dawes to warn the militia and the two men on 18 April. As a result, the militia was prepared to face Pitcairn the following day, and the Battles of Lexington and Concord broke out, marking the beginning of the American Revolutionary War.
Soon after, the colonial militias besieged Boston. Gage ordered that fortifications along Boston Neck and the Southgate Fort were expanded and strengthened, becoming known as "Gage's Lines". He also issued a pardon to all rebels that would lay down their arms, with the exception of Adams and Hancock.
In an attempt to break the siege, Gage and other generals planned to take Dorchester Heights and Charlestown. The rebels were notified of this, and William Prescott built fortifications at Breed's Hill. In the ensuing Battle of Bunker Hill, the British forced the rebels to retreat, although the former took great casulties. One of these was Pitcairn, who was assassinated. Gage was recalled, and William Howe took over the situation. The British evacuated Boston in March 1776, after Henry Knox had placed artillery at Dorchester Heights.