On September 26, 1815, Alexander I of Russia, Francis I of Austria and Frederick William III of Prussia signed a short document in Paris, committing themselves to act on the principles of Christian charity, peace and justice and to regard one another as brothers. It was the Holy Alliance. All the monarchs of Europe except the British regent, the Pope and the Sultan eventually signed it.
It was Alexander's idea and it was not a treaty in the ordinary sense. The text is about religion and the duties of princes rather than about territory, boundaries or force, and it binds the signatories to nothing measurable. Metternich, who read it and thought it harmless nonsense, drafted a version that removed the parts he disliked. Castlereagh, the British foreign secretary, advised the Prince Regent not to sign, and described it privately as a piece of sublime mysticism and nonsense.
It came from that mess. Napoleon had been defeated, the Congress of Vienna had redrawn Europe, and the powers had agreed in the Quadruple Alliance of November 1815 to meet periodically to maintain the settlement. That arrangement, the congress system, is the thing that mattered, and the Holy Alliance is what it became known by, inaccurately.
Alexander I was a complicated figure: an autocrat who had talked of constitutions, who had been influenced by pietist and mystic writing, and who had also been the man who had carried the war to Paris. The document reflects that. It also reflects the conservative reading of the previous 25 years, in which revolution had come out of irreligion, and the answer to revolution was therefore a restoration of thrones and altars.
The system worked for about seven years and then broke on Greece and Latin America. At Troppau in 1820 and Laibach in 1821 the eastern powers agreed to intervene against revolutions in Naples and Piedmont. At Verona in 1822 they agreed to let France intervene in Spain. Britain withdrew from the process at that point, and Canning, who succeeded Castlereagh in 1822, refused to attend.
The intervention in Greece did not happen in time to save the Ottoman position there, and the Greeks won their independence with British, French and Russian help in 1830. The Latin American colonies got their independence largely because Britain would not let the alliance act. The Holy Alliance itself was never formally dissolved, and it is generally counted as dead by 1830.