On 19 April 1775 a detachment of some 800 British regular soldiers, dispatched from Boston by the governor of Massachusetts, Thomas Gage, to destroy a militia weapons magazine at Concord, were fired on by a local militia company that had assembled on Lexington village green. The militia refused to disperse, and from somewhere a shot was fired. Men on both sides opened fire, believing themselves to be under attack. Within thirty or forty seconds eight militiamen were dead, and a regular wounded. The militia quickly dispersed and the regulars moved on to Concord where a second clash occurred while the magazine was destroyed. As the British regulars began their return to Boston, they came under sustained attack from a growing number of enraged inhabitants. Many had been roused by the sight of smoke rising from the arsenal at Concord, for it appeared as if the regulars were burning the village. By early afternoon more than a thousand armed colonists had emerged. For fifteen miles back to Boston the regulars were harassed by a hidden enemy firing at them from windows and behind walls. Only the timely arrival of reinforcements saved the original detachment, as the British losses amounted to 273 men killed, injured, and missing.

The news of the skirmishes quickly spread, men called to arms by ‘alarm Guns’ and ‘Expresses’ carried by post riders. ‘Expresses are hastening from town to town, in all directions through the country,’ wrote a Massachusetts physician, ‘spreading the melancholy tidings and inspiriting and rousing the people To Arms! To Arms!’ New Englanders responded with the same behaviour as they had done following several previous alarms that had occurred over recent months: they picked up their guns and marched to Boston. ‘We had about 130 men march Towards Boston,’ wrote a farmer in Hingham, Massachusetts, ‘the Rest of ye Town under Arms on account of the Express we had yesterday.’ It was not uncommon to hear stories circulated that men had ‘left their farms, taken their muskets, and gone to Boston’. Sylvanus Wood recalled how he heard the church bells, and ‘waited for no man’ as he went off with a friend to join the militia company. Another wrote how his son came home intending ‘to Sett off for our army to morrow morning’. Within a few days the regulars were besieged by nearly 16,000 men encamped outside the city.