In September 1775, early in the American Revolutionary War, Colonel Benedict Arnold led a force of 1,100 Continental Army troops on an expedition from Cambridge in the Province of Massachusetts Bay to the gates of Quebec City. The expedition was part of a two-pronged invasion of the British Province of Quebec, and passed through the wilderness of what is now Maine. The other expedition invaded Quebec from Lake Champlain, led by Richard Montgomery.

Unanticipated problems beset the expedition as soon as it left the last significant colonial outposts in Maine. The portages up the Kennebec River proved grueling, and the boats frequently leaked, ruining gunpowder and spoiling food supplies. More than a third of the men turned back before reaching the height of land between the Kennebec and Chaudire rivers. The areas on either side of the height of land were swampy tangles of lakes and streams, and the traversal was made more difficult by bad weather and inaccurate maps. Many of the troops lacked experience handling boats in white water, which led to the destruction of more boats and supplies in the descent to the Saint Lawrence River via the fast-flowing Chaudire.

By the time that Arnold reached the settlements above the Saint Lawrence River in November, his force was reduced to 600 starving men. They had traveled about 350 miles (560 km) through poorly charted wilderness, twice the distance that they had expected to cover. Arnold's troops crossed the Saint Lawrence on November 13 and 14, assisted by the local French-speaking Canadiens, and attempted to put Quebec City under siege. Failing in this, they withdrew to Point-aux-Trembles until Montgomery arrived to lead an unsuccessful attack on the city. Arnold was rewarded for his effort in leading the expedition with a promotion to brigadier general.

Arnold's route through northern Maine has been listed on the National Register of Historic Places as the Arnold Trail to Quebec, and some geographic features in the area bear names of expedition participants.

The American Revolutionary War (April 19, 1775 – September 3, 1783), also known as the Revolutionary War or American War of Independence, secured a United States of America independent from Great Britain. Fighting began on April 19, 1775, followed by the Declaration of Independence on July 4, 1776. The American Patriots were supported by France and Spain, conflict taking place in North America, the Caribbean, and Atlantic Ocean. It ended on September 3, 1783 when Britain accepted American independence in the Treaty of Paris, while the Treaties of Versailles resolved separate conflicts with France and Spain.Established by Royal charter in the 17th and 18th centuries, the American colonies were largely autonomous in domestic affairs and commercially prosperous, trading with Britain and its Caribbean colonies, as well as other European powers via their Caribbean entrepôts. After British victory in the Seven Years' War in 1763, tensions arose over trade, colonial policy in the Northwest Territory and taxation measures, including the Stamp Act and Townshend Acts. Colonial opposition led to the 1770 Boston Massacre and 1773 Boston Tea Party, with Parliament responding by imposing the so-called Intolerable Acts.

On September 5, 1774, the First Continental Congress drafted a Petition to the King and organized a boycott of British goods. Despite attempts to achieve a peaceful solution, fighting began with the Battle of Lexington on April 19, 1775 and in June Congress authorized George Washington to create a Continental Army. Although the "coercion policy" advocated by the North ministry was opposed by a faction within Parliament, both sides increasingly viewed conflict as inevitable. The Olive Branch Petition sent by Congress to George III in July 1775 was rejected and in August Parliament declared the colonies to be in a state of rebellion.

Following the loss of Boston in March 1776, Sir William Howe, the new British commander-in-chief, launched the New York and New Jersey campaign. He captured New York City in November, before Washington won small but significant victories at Trenton and Princeton, which restored Patriot confidence. In summer 1777, Howe succeeded in taking Philadelphia, but in October a separate force under John Burgoyne was forced to surrender at Saratoga. This victory was crucial in convincing powers like France and Spain an independent United States was a viable entity.

France provided the US informal economic and military support from the beginning of the rebellion, and after Saratoga the two countries signed a commercial agreement and a Treaty of Alliance in February 1778. In return for a guarantee of independence, Congress joined France in its global war with Britain and agreed to defend the French West Indies. Spain also allied with France against Britain in the Treaty of Aranjuez (1779), though it did not formally ally with the Americans. Nevertheless, access to ports in Spanish Louisiana allowed the Patriots to import arms and supplies, while the Spanish Gulf Coast campaign deprived the Royal Navy of key bases in the south.

This undermined the 1778 strategy devised by Howe's replacement, Sir Henry Clinton, which took the war into the Southern United States. Despite some initial success, by September 1781 Cornwallis was besieged by a Franco-American force in Yorktown. After an attempt to resupply the garrison failed, Cornwallis surrendered in October, and although the British wars with France and Spain continued for another two years, this ended fighting in North America. In April 1782, the North ministry was replaced by a new British government which accepted American independence and began negotiating the Treaty of Paris, ratified on September 3, 1783.