The history of the United Kingdom began in the early eighteenth century with the Treaty of Union and Acts of Union. The core of the United Kingdom as a unified state came into being in 1707 with the political union of the kingdoms of England and Scotland, into a new unitary state called Great Britain. Of this new state of Great Britain the historian Simon Schama said:
What began as a hostile merger would end in a full partnership in the most powerful going concern in the world... it was one of the most astonishing transformations in European history.

— Simon Schama,
The Act of Union 1800 added the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland.

The first decades were marked by Jacobite risings which ended with defeat for the Stuart cause at the Battle of Culloden in 1746. In 1763, victory in the Seven Years' War led to the growth of the First British Empire. With defeat by the United States, France and Spain in the War of American Independence, Britain lost its 13 American colonies and rebuilt a Second British Empire based in Asia and Africa. As a result, British culture, and its technological, political, constitutional, and linguistic influence, became worldwide. Politically the central event was the French Revolution and its Napoleonic aftermath from 1793 to 1815, which British elites saw as a profound threat, and worked energetically to form multiple coalitions that finally defeated Napoleon in 1815. The Tories, who came to power in 1783, remained in power until 1830. Forces of reform, often emanating from the Evangelical religious elements, opened decades of political reform that broadened the ballot, and opened the economy to free trade. The outstanding political leaders of the 19th century included Palmerston, Disraeli, Gladstone, and Salisbury. Culturally the Victorian era was a time of prosperity and dominant middle-class virtues when Britain dominated the world economy and maintained a generally peaceful century, 1815–1914. The First World War , with Britain in alliance with France, Russia and the United States, was a furious but ultimately successful total war with Germany. The resulting League of Nations was a favourite project in Interwar Britain. However, while the Empire remained strong, as did the London financial markets, the British industrial base began to slip behind Germany and, especially, the United States. Sentiments for peace were so strong that the nation supported appeasement of Hitler's Germany in the late 1930s, until the Nazi invasion of Poland in 1939 started the Second World War. In the Second World War, the Soviet Union and the U.S. joined Britain as the main Allied powers.