The Boston campaign was the opening campaign of the American Revolutionary War. The campaign was primarily concerned with the mobilization of Patriot 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 militias turned out according to plan to interdict and harass the British attempt to seize military stores and leaders in Concord, Massachusetts. The entire British expedition suffered significant casualties during a running battle back to Charlestown against an ever-growing number of militia.