General Thomas Gage, an amiable English gentleman with an American-born wife, commanded the garrison at Boston, where political activity had almost wholly replaced trade. Gage's main duty in the colonies had been to enforce the Coercive Acts. When news reached him that the Massachusetts colonists were collecting powder and military stores at the town of Concord, 32 kilometers away, Gage sent a strong detail from the garrison to confiscate these munitions.  

After a night of marching, the British troops reached the village of Lexington on April 19, 1775, and saw a grim band of 70 Minutemen--so named because they were said to be ready to fight in a minute--through the early morning mist. The Minutemen intended only a silent protest, but Major John Pitcairn, the leader of the British troops, yelled, "Disperse, you damned rebels! You dogs, run!" The leader of the Minutemen, Captain John Parker, told his troops not to fire unless fired at first. The Americans were withdrawing when someone fired a shot, which led the British troops to fire at the Minutemen. The British then charged with bayonets, leaving eight dead and 10 wounded. It was, in the often quoted phrase of Ralph Waldo Emerson, "the shot heard 'round the world."  

Then the British pushed on to Concord. The Americans had taken away most of the munitions, but the British destroyed whatever was left. In the meantime, American forces in the countryside mobilized, moved toward Concord and inflicted casualties on the British, who began the long return to Boston. All along the road, however, behind stone walls, hillocks and houses, militiamen from "every Middlesex village and farm" made targets of the bright red coats of the British soldiers. By the time the weary soldiers stumbled into Boston, they suffered more than 250 killed and wounded. The Americans lost 93 men.  

While the alarms of Lexington and Concord were still resounding, the Second Continental Congress met in Philadelphia, Pennsylvania, on May 10, 1775. By May 15, the Congress voted to go to war, inducting the colonial militias into continental service and appointing Colonel George Washington of Virginia as commander-in-chief of the American forces. In the meantime, the Americans would suffer high casualties at Bunker Hill just outside Boston. Congress also ordered American expeditions to march northward into Canada by fall. Although the Americans later captured Montreal, they failed in a winter assault on Quebec, and eventually retreated to New York.  

Despite the outbreak of armed conflict, the idea of complete separation from England was still repugnant to some members of the Continental Congress. In July, John Dickinson had drafted a resolution, known as the Olive Branch Petition, begging the king to prevent further hostile actions until some sort of agreement could be worked out. The petition fell on deaf ears, however, and King George III issued a proclamation on August 23, 1775, declaring the colonies to be in a state of rebellion.