The siege of Boston (April 19, 1775 March 17, 1776) was the opening phase of the American Revolutionary War. New England militiamen prevented the movement by land of the British Army, which was garrisoned in what was then the peninsular town of Boston, Massachusetts Bay. Both sides had to deal with resource, supply, and personnel issues over the course of the siege. British resupply and reinforcement was limited to sea access, which was impeded by American vessels. The British abandoned Boston after eleven months and transferred their troops and equipment to Nova Scotia.

The siege began on April 19 after the Battles of Lexington and Concord, when Massachusetts militias blocked land access to Boston. The Continental Congress formed the Continental Army from the militias involved in the fighting and appointed George Washington as Commander in Chief. In June 1775, the British seized Bunker and Breed's Hills, from which the Continentals were preparing to bombard the city, but their casualties were heavy and their gains insufficient to break the Continental Army's control over land access to Boston. After this the Americans laid siege to the city; no major battles were fought during this time and the conflict was limited to occasional raids, minor skirmishes, and sniper fire. British efforts to supply their troops were significantly impacted by the smaller but more agile American forces operating on both land and sea and the British consequently suffered from a continual lack of food, fuel, and supplies during the siege.

In November 1775, George Washington sent Henry Knox on a mission to bring to Boston the heavy artillery that had recently been captured at Fort Ticonderoga. In a technically complex and demanding operation, Knox was able to bring the cannons to Boston in January 1776 and by March this artillery had fortified Dorchester Heights which overlooked Boston harbor. This development threatened to cut off the British supply lifeline from the sea. British commander William Howe saw his position as indefensible and on March 17 he withdrew his forces from Boston to Halifax, Nova Scotia.

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.