The Holy Alliance was a coalition of Russia, Austria and Prussia created in 1815 at the behest of Tsar Alexander I of Russia, ostensibly to uphold Christianity in European political life but in practice as a bastion against revolution. In time other European nations joined as well.
The Holy Alliance was, in a manner of speaking, the first modern international peacekeeping organization, though it was rooted in antiquated models of politics and reflected the return to conservative politics in Europe after the long struggle against Revolutionary and Imperial France.
It was subsequently acceded to by all the monarchs of Europe except the King of Great Britain, who declined to sign on constitutional grounds, Pope Pius VII, who refused to treat with Protestant monarchs, and the Sultan of Turkey.
The Alliance is usually associated with the Quadruple and Quintuple Alliances, which included the United Kingdom and (from 1818) France with the aim of upholding the European peace settlement concluded at the Congress of Vienna. The Alliance was conventionally taken to have become defunct with Alexander's death in 1825.