George III (George William Frederick) was King of Great Britain and Ireland from 25 October 1760 until the union of the two countries on 1 January 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 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 Britain's 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.
Although the Holy Roman Empire was dissolved in 1806, George III's government did not consider the dissolution to be final, and he continued to be styled "Duke of Brunswick-Luneburg, Arch-treasurer and Prince-Elector of the Holy Roman Empire" until 1814. In 1813, George III was restored to his Hanoverian territories, and in October 1814 they were constituted as the independent Kingdom of Hanover at the Congress of Vienna.
In the later part of his life, George III had recurrent, and eventually permanent, mental illness. Although it has since been suggested that he had 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.