News of the fighting at Concord and Lexington traveled swiftly. A dispatch rider named Israel Bissel left Watertown, Massachusetts, at ten o'clock on the morning of April 19. He carried word of the fighting through Connecticut, slowed only by the need to find fresh horses. He arrived in New York City on April 23. Another night and day of hard riding took him to Philadelphia. Bissel was not alone. Messengers reached Annapolis, Maryland, on April 27; Edenton, North Carolina, on May 4; and Charleston, South Carolina, on May 10. The Patriot networks and institutions created over the years to resist British policy immediately took action. British authority in the colonies began to unravel.

Up and down the colonies, British governors found themselves in an impossible situation. Real power was seized by the colonial legislatures. The political initiative lay with the Patriots. When a new royal governor arrived at Charleston in June 1775, he was politely greeted by a handful of officials and ignored by the public. After three months of watching helplessly as the South Carolinians prepared to resist the King's forces, the governor retreated to a British warship, never to return.

Sometimes the Americans were more forceful in ejecting their governors. In Virginia, the House of Burgesses compelled Lord Dunmore to leave. Dunmore would go on to organize loyalist forces to contest control of the colony, as would Josiah Martin in North Carolina. William Tryon, the royal governor of New York, served as a general in the British army during the war. But no matter how militantly defiant they were, soon the British governors were all refugees, fleeing to the shelter of the Royal Navy.

Lake Champlain provided a strategically vital invasion route between the colonies and Canada. The lake and connecting rivers formed a convenient highway through the wilderness. During the French and Indian War, it had been a major theater of conflict, and the strongholds that both sides built on it became famous. One of these was the French Fort Ticonderoga. Captured by the British, it had been allowed to deteriorate after the war, but the fort retained its strategic significance and was well stocked with artillery and other military supplies. Late in April, the Massachusetts Committee of Safety commissioned Benedict Arnold to collect a force and capture Ticonderoga. Arnold was a young merchant and sea captain from Connecticut who had already distinguished himself as a militia officer. He had only begun to recruit when he learned that Ethan Allen, leader of Vermont's Green Mountain Boys, was planning on attacking the fort himself. Arnold rushed to Allen's camp and claimed command, but Allen was too strongwilled to yield to the stranger. Arnold had to accompany Allen's eighty-three-man force as a volunteer.

The Americans took the forty-two-man garrison of Ticonderoga by surprise on May 10. Allen famously demanded the surrender of the British commander “in the name of the Great Jehovah and the Continental Congress.” A force of Green Mountain Boys under Seth Warner captured Crown Point, another debilitated British stronghold, from another tiny and bewildered garrison. rnold briefly occupied St. John's in Canada, taking a few more British soldiers prisoner. For the moment, the Americans controlled Lake Champlain.