After the Greek state's declaration of independence, it took more than thirty years for its borders to be expanded for the first time. This expansion was neither a result of any of the liberation revolts of the Greek state, nor was it brought about to the detriment of the Ottoman Empire, the only enemy Greece had until 1878. With the treaty signed on 17th-29th March 1864 between the three Powers - England, France and Russia - and the Greek Kingdom, the Ionian Islands came definitively under Greek rule on 21st May. This evolution completed a series of deliberations and diplomatic negotiations, which imposed several relatively onerous conditions on Greece, which was excluded from most diplomatic movements.

The treaty confirmed perpetual neutrality for Corfu, which was why part of the fortifications of the city and of Paxoi were pulled down. The Greek state accepted all the obligations towards foreign governments, associations and civilians derived from conventions contracted with the Ionian State or the Protective Power, Great Britain. This regulation regarded the public debt of the Ionian Islands, commercial and maritime privileges of foreigners and mostly the right of issue of the Ionian Bank. The Greek state also committed itself to indemnify and provide pensions to English employees who would lose their jobs after the Union. The prevalence of the Orthodox Church was recognized but at the same time religious and devotional freedom for all beliefs was declared and the privileges of the Catholic Church were maintained. Great Britain withdrew protection from the Ionian Islands and, along with France and Russia, extended the guarantees pertaining to Greece and the Ionian Islands.

On 23rd September-13th October the Ionian Parliament realized the aim of its convocation by deciding to proceed with the Union with Greece, forming 'a single and undivided state under the constitutional sceptre of George A'. This development caused some bitterness among a faction of the Radicals, who had fought for a subversive and revolutionary Union with an independent and democratic Greece.