Lochry's Defeat, also known as the Lochry massacre, was a battle fought on August 24, 1781, near present-day Aurora, Indiana, in the United States. The battle was part of the American Revolutionary War (17751783), which began as a conflict between Great Britain and the Thirteen Colonies before spreading to the western frontier, where American Indians entered the war as British allies. The battle was short and decisive: about one hundred Indians of local tribes led by Joseph Brant, a Mohawk military leader who was temporarily in the west, ambushed a similar number of Pennsylvania militiamen led by Archibald Lochry. Brant and his men killed or captured all of the Pennsylvanians without suffering any casualties.

Lochry's force was part of an army being raised by George Rogers Clark for a campaign against Detroit, the British regional headquarters. Clark, the preeminent American military leader on the northwestern frontier, worked with Governor Thomas Jefferson of Virginia in planning an expedition to capture Detroit, by which they hoped to bring an end to British support of the Indian war effort. In early August 1781, Clark and about 400 men left Fort Pitt in Pennsylvania by boat, floating down the Ohio River a few days ahead of Lochry and his men, who were trying to catch up.

Joseph Brant's force was part of a combined British and Indian army being raised to counter Clark's offensive. Brant had too few men to challenge Clark, but when he intercepted messengers traveling between Clark and Lochry, he learned about Lochry's smaller group bringing up the rear. When Lochry landed to feed his men and horses, Brant launched his overwhelmingly successful ambush. Because Clark had been able to recruit only a fraction of the men he needed for his campaign, the loss of Lochry's men resulted in the cancellation of Clark's expedition.

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.