The Boston campaign was the opening campaign of the American Revolutionary War.
The campaign was primarily concerned with the formation of American colonial irregular militia units, and their transformation into a unified Continental Army. The campaign's military conflicts started with the Battles of Lexington and Concord on April 19, 1775, in which colonial militias mustered to defend against the seizure of military stores in Concord, Massachusetts by British Army regulars. Some British units were defeated in a confrontation at Concord's North Bridge, and the entire British expedition suffered significant casualties during a running battle back to Charlestown against an ever-growing number of colonial militia.
Before the war, Boston had been the scene of much revolutionary activity, leading to the Massachusetts Government Act that ended home rule as a punishment in 1774. Popular resistance to these measures, however, compelled the newly appointed royal officials in Massachusetts to resign or to seek refuge in Boston. Lieutenant General Thomas Gage, the British North American commander-in chief, commanded four regiments of British regulars (about 4,000 men) from his headquarters in Boston, but the countryside was in the hands of the Revolutionaries.
Wikipedia: Boston Campaign
Wikipedia: American Revolutionary War