Splendid isolation is a term used to describe the 19th-century British diplomatic practice of avoiding permanent alliances from 1815 to 1902. The concept developed as early as 1822, when Britain left the post-1815 Concert of Europe, and continued until the 1902 Anglo-Japanese Alliance and the 1904 Entente Cordiale with France. As Europe was divided into two power blocs, Britain became aligned with the French Third Republic and the Russian Empire (known as the Triple Entente) against the German Empire, Austria-Hungary and the Kingdom of Italy (The Triple Alliance).

The term was coined in January 1896 by a Canadian politician, George Eulas Foster. He indicated his approval for Britain's minimal involvement in European affairs by saying "In these somewhat troublesome days when the great Mother Empire stands splendidly isolated in Europe."

There is considerable historical debate over the extent to which this approach was intentional or accidental, its impact, or even if it ever existed, other than as a useful phrase.

Nineteenth-century British foreign policy was characterised by a reluctance to enter into permanent alliances with other Great Powers. Often assumed to apply only to the latter part of the century, some historians argue it originated after the 1822 Congress of Verona, when Britain withdrew from the post-1815 Concert of Europe, guided by Foreign Secretary George Canning. His principles dominated British foreign policy for decades, and have been summarised as follows;

Non-intervention; no European police system; every nation for itself, and God for us all; balance of power; respect for facts, not for abstract theories; respect for treaty rights, but caution in extending them...England not Europe...Europe's domain extends to the shores of the Atlantic, England's begins there.

For much of the 19th century, Britain sought to maintain the existing balance of power in Europe, while protecting trade routes to its colonies and dominions, especially those connecting to British India through the Suez Canal. In 1866, the Foreign Secretary Lord Derby explained this policy as follows:

It is the duty of the Government of this country, placed as it is with regard to geographical position, to keep itself upon terms of goodwill with all surrounding nations, but not to entangle itself with any single or monopolising alliance with any one of them; above all to endeavour not to interfere needlessly and vexatiously with the internal affairs of any foreign country.

One exception was the 1839 Treaty of London, recognising the independence of Belgium, which led to Britain entering World War I in 1914. The ports of Ostend, Antwerp and Zeebrugge were so important to ensuring control of the English Channel, Britain guaranteed Belgian independence, by military means if required.