George III (George William Frederick; June 4, 1738 – January 29, 1820) was King of Great Britain and King of Ireland from October 25, 1760 until the union of these two countries on January 1, 1801, after which he was King of the United Kingdom of Great Britain and Ireland until his death. He was concurrently Duke and prince-elector of Brunswick-Lüneburg ("Hanover") in the Holy Roman Empire until his promotion to King of Hanover on 12 October 1814. He was the third British monarch of the House of Hanover, but unlike his two Hanoverian predecessors he was born in Britain, spoke English as his first language, and never visited Hanover.
His life and reign, which were longer than any other British monarch before him, were marked by a series of military conflicts involving his kingdoms, much of the rest of Europe, and places farther afield in Africa, the Americas and Asia. 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. Further wars against revolutionary and Napoleonic France from 1793 concluded in the defeat of Napoleon at the Battle of Waterloo in 1815.
In the later part of his life, George III suffered from recurrent, and eventually permanent, mental illness. Although it has since been suggested that he suffered from the blood disease porphyria, the cause of his illness remains unknown. After a final relapse in 1810, a regency was established, and George III's eldest son, George, Prince of Wales, ruled as Prince Regent. On George III's death, the Prince Regent succeeded his father as George IV.
Historical analysis of George III's life has gone through a "kaleidoscope of changing views" that have depended heavily on the prejudices of his biographers and the sources available to them. Until re-assessment occurred during the second half of the twentieth century, his reputation in the United States was one of a tyrant and in Britain he became "the scapegoat for the failure of imperialism".