George III was King of Great Britain and King of Ireland from 25 October 1760 until 1 January 1801, and thereafter of the United Kingdom, formed by the union of Great Britain and Ireland, until his death. He was concurrently Duke of Brunswick-Lüneburg, and thus Prince-Elector of Hanover in the Holy Roman Empire, until he became King of Hanover on 12 October 1814. He was the third British monarch of the House of Hanover, and the first of Hanover to be born in Britain and speak English as his first language. In fact, he never visited Germany.

George III's long reign was marked by a series of military conflicts involving his kingdom and much of the rest of Europe. Early in his reign, Great Britain defeated France in the Seven Years' War, becoming the dominant European power in North America and India. However, many of its American colonies were soon lost in the American Revolutionary War, which led to the establishment of the United States. Later, the kingdom became involved in a series of wars against revolutionary and Napoleonic France, which finally concluded in the defeat of Napoleon in 1815.