In a military sense, the Battles of Lexington and Concord comprise one battle, but in popular conception, the American Revolution began between 5:30 and 6:00 a. m. on April 19, 1775, at Lexington Green.
During the early morning hours of April 19, about 700 British troops from Boston marched to Lexington, Massachusetts, to find and confiscate some ammunition rumored to be nearby in Concord. As they approached Lexington Green, where they found about 75 American militiamen drawn up, a British officer, Lieutenant William Sutherland, reported that
. . . a fellow from the corner of the road . . . cocked his piece at me, but burnt priming. . . . We still went on further when three shots more were fired at us, which we did not return. . . . (The Fire of Liberty. London: The Folio Society, 1983. Page 23)
An American on the Green reported at the same time that a British soldier fired at the colonists but that his rifle also mis-fired. To this day, no historian has been able to determine with certainty which side fired first, but in the event, who fired does not matter. What matters, of course, is that someone fired.
After up to three volleys of fire from both sides, up to eight (accounts vary) Americans had been killed, seven wounded, and one British soldier was slightly wounded. This incident began a "running battle" between the Americans and British that continued as the British moved on to Concord and the ammunition cache.
By the time the British troops reached Concord, the ammunition had been taken away but more armed colonists had arrived. When the British began destroying the small amount of ammunition they found, the Americans assumed they were setting the town on fire. At Concord's North Bridge, a British platoon was holding the bridge for the main body of troops and, as they became surrounded by militiamen, fired at them to push them away from the bridge--this is the actual "shot" that is credited with starting the Revolution.
When the British at Concord returned to Lexington, they found a newly-arrived contingent of British troops from Boston, but they also found up to 2,000 Americans who had descended on Lexington over the morning hours. As the British began the 18-mile march back to Boston, the Americans began shooting in earnest, and this running battle lasted until the British returned to Cambridge, resulting in about 250 British casualties and perhaps 100 Americans killed and wounded.
From a military standpoint, the battle had little significance. From a psychological and political standpoint, however, the effect was profound. Once blood was spilled, many Americans felt that the ties that bound them to Great Britain were irrevocably broken. Perhaps more important was the realization that the Americans could successfully fight the British Army, world-renowned as the best army in 18thC. Europe, but now shown to be vulnerable to an un-trained but well-motivated group of American militiamen.