The Union of the Ionian Islands with Greece took place with the ratification of the Treaty of London on May 21, 1864.

Britain ceded the United Kingdom of the Ionian Islands to Greece.

The United Kingdom of the Ionian Islands included Corfu, Ithaca, Paxos, Kefalonia, Zakynthos, Santa Maura (Lefkada) and Tsirigo (Kythera)

The Ionian Islands had been her possession since 1815, given by France, which, in turn, inherited them from Russia and the Ottoman Empire for the second time.

The guarantors of the agreement were the Kingdom of Greece, the Kingdom of Prussia, the Russian Empire and the United Kingdom.

Ever since Greece became an independent state, the inhabitants of the Ionian Islands have been lobbying the British for union with Greece. At a cabinet meeting in 1862, British Foreign Secretary Palmerston decided to cede the Ionian Islands to Greece. This policy was also promoted by Queen Victoria. The main reason was that it was extremely expensive to maintain. Furthermore, the islands were of little strategic importance.

The decision to cede the islands was also influenced by the assumption of the Greek throne by the Danish Prince George, a devoted Anglophile. In a referendum in November 1862, the Greeks had elected Queen Victoria’s second son, Prince Alfred, as their King, hoping to get the Ionian Islands.

After negotiations, the Treaty of London was signed by the Greek representative, Charilaos Trikoupis on March 29, 1864. On May 2, 1864, the British left and the Ionian Islands became three provinces of the Kingdom of Greece. Britain retained rights to use the port of Corfu.

This can be seen as the first example of voluntary decolonization by Britain.

Source: Wikipedia