Which empire did Greece declare its independence from in 1821?

A secret organization called the Filiki Eteria had been founded in 1814 with the intent of liberating Greece from the Ottoman Empire. By 1821 the revolt began in the Peloponnese peninsula. The uprising then spread to Crete, Macedonia and Central Greece. An improvised Greek navy scored early success against the Ottoman fleet, preventing re-reinforcements from reaching Greece.

However, the Ottoman Sultan contacted the Egyptians for aid against the uprising. Egyptian forces landed in the Peloponnese in 1825 and by the end of the year most of the peninsula was under Egyptian control. European powers (Britain, France and Russia) after years of negotiations with the rebels agreed to intervene in the struggle. Each country sent naval vessels which met the Ottoman-Egyptian fleet in battle at Navarino in 1827, resulting in the destruction of the Ottoman-Egyptian fleet.

By 1828, a French expeditionary force landed in Greece, prompting the Egyptian forces to withdraw and the Ottoman garrison forces surrender. This reversal allowed for the Greek rebels to again attack Central Greece. By May 1832 after negotiations between Britain, France and Russia on the side of Greek independence and the Ottoman Empire, Greece was recognized as an independent country.