The Treaty of Alliance (French: trait d'alliance (1778)), also known as the Franco-American Treaty, was a defensive alliance between the Kingdom of France and the United States of America formed amid the American Revolutionary War with Great Britain. It was signed by delegates of King Louis XVI and the Second Continental Congress in Paris on February 6, 1778, along with the Treaty of Amity and Commerce and a secret clause providing for the entry of other European allies; together these instruments are sometimes known as the Franco-American Alliance or the Treaties of Alliance. The agreements marked the official entry of the United States on the world stage, and formalized French recognition and support of U.S. independence that was to be decisive in America's victory.

The Treaty of Alliance was signed immediately after the Treaty of Amity and Commerce, in which France was the first nation to formally recognize the U.S. as a sovereign nation; this treaty had also established mutual commercial and navigation rights between the two nations, in direct defiance of the British Acts of Trade and Navigation, which restricted American access to foreign markets. In contemplation that these commercial and diplomatic ties would result in hostilities between France and Britain, the Treaty of Alliance guaranteed French military support in just such an event. It also forbade either nation from making a separate peace with Britain, and was contemplated as a permanent defensive pact.

The successful negotiation of the Treaty of Alliance and its sister agreements is considered the "single most important diplomatic success of the colonists", since it helped secure vital aid in the war with Britain; the treaties were immediately followed by substantial material, military, and financial support to the American cause. Some historians consider the signing of the Treaty of Alliance as marking America's de jure recognition as an independent nation. Notwithstanding its significance, subsequent complications with the Treaty of Alliance led to its annulment by the turn of the 19th century, with the United States eschewing formal military alliances until the Second World War.

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.