The Treaty of Constantinople of 2 April [O.S. 21 March] 1800 was concluded between the Ottoman Empire and the Russian Empire, and heralded the creation of the Septinsular Republic, the first autonomous Greek state since the Fall of the Byzantine Empire.
The new state comprised the Ionian Islands, seven islands off the western coast of Greece, that had been under Venetian rule for centuries, and thus had escaped Ottoman conquest, unlike the Greek mainland. Following the Fall of the Republic of Venice in 1797, the islands had come under French rule. Initially popular, the French quickly alienated the Greeks with their anti-clerical policies, and especially the islands' native nobility, with their republican ideals. In 1798, the Russians and Ottomans launched a joint expedition against the French-held islands, culminating in the capture of Corfu in 1799. In the aftermath, representatives of the islands' nobility went to the Ottoman capital, Constantinople, and the Russian capital, Saint Petersburg, to secure the island's self-governance. The resulting negotiations led to the creation of the Septinsular Republic as a federal republic under the suzerainty of the Ottoman sultan. A conservative and reactionary constitution, the so-called "Byzantine Constitution", was issued at the same time, and enshrined the dominance of the nobility in the Islands' affairs. In practice, the islands remained under Russian rather than Ottoman influence, and domestic political developments resulted in the adoption of a new constitution as early as 1801. The Septinsular Republic itself would survive until 1807, when it was annexed to Napoleon's French Empire under the Treaty of Tilsit.