Yes, the Revolutionary War started on the 19th, but spot on with your thought.  This info is from a Paul Revere page: On the evening of April 18, 1775, Paul Revere was summoned by Dr. Joseph Warren of Boston and given the task of riding to Lexington, Massachusetts, with the news that regular troops were about to march into the countryside northwest of Boston. According to Warren, these troops planned to arrest Samuel Adams and John Hancock, who were staying at a house in Lexington, and probably continue on to the town of Concord, to capture or destroy military stores — gunpowder, ammunition, and several cannon — that had been stockpiled there (in fact, the British troops had no orders to arrest anyone — Dr. Warren’s intelligence on this point was faulty). Revere contacted an unidentified friend (probably Robert Newman, the sexton of Christ Church in Boston’s North End) and instructed him to show two lanterns in the tower of Christ Church (now called the Old North Church) as a signal in case Revere was unable to leave town. The two lanterns meant that the British troops planned to row “by sea” across the Charles River to Cambridge, rather than march “by land” out Boston Neck.