Why was the battle of Lexington and concord started?

British General Gage learned that the colonists had stored supplies and ammunition at Concord, MA, some 20 miles from Boston. On 19 April 1775, he sent 700 of his troops to seize the munitions and, if possible, capture and arrest Samuel Adams and John Hancock. Gage had received information that those two "rebels" were hiding out in Concord. Warning the Americans that the British were marching to Concord were the trio Revere, Dawes, and Prescott, of the famous "Midnight Ride." When British Major John Pitcairn arrived at Lexington, which was on the way to Concord, he found 70 American minutemen (actually, they were almost all farmers) in battle formation at the town square. Pitcairn ordered the Americans to disperse but when they didn't move after the second order to do so, someone fired a shot. It is not known if the shot was fired by Americans or British. The British easily cleared Lexington and marched on to Concord. He found more Americans arming the bridge into the town so Pitcairn order the British to return to Boston. All the way back to Boston, the Americans sniped at the British from behind trees and rocks, inflicting serious injury to the British troops. When the Redcoats reached Boston, 250 had been killed or wounded.

The american revolutionaries were trying to stop a advancing force of British troops. Which is why there is a trail that follows the british march through the two cities