The Treaty of Karlowitz (1699) marked the end of the Ottoman Empire’s tenure as the heavyweight military power in Eastern Europe (though it still remained competitive). Then passed several decades where the Ottomans won some, probably lost more, but still sort of held their own. But the Treaty of Küçük Kaynarca, signed on this date in 1774, marked the point where European powers, rightly or wrongly, began to think, hey, not only are these Ottomans no longer a power, they’re actually pretty easy pickings, all things considered. If you’ve ever heard of the late Ottoman Empire being referred to as the “sick man of Europe,” this is the point when that meme begins to take hold. The snootier academic way to refer to this is as the “Eastern Question.” France, Britain, and Austria were doing the asking, and the “question,” specifically, was “how in the hell are we going to keep Russia from swallowing the Ottoman Empire up whole?”
Küçük Kaynarca, signed in the city of the same name (which is now the city of Kaynardzha, Bulgaria) signified the end of the Russo-Turkish War of 1768-1774, which was fought primarily in and around the Black Sea. That war ended with the Russians occupying Wallachia, Moldavia, and parts of Crimea and with the Ottomans, well, no longer occupying any of those places. Oh, and Russia pretty much destroyed the Ottomans’ Black Sea fleet to boot. It was a thorough Russian victory, so thorough that the theretofore tenuously enthroned Empress of Russia, Catherine something-or-other, was able to use this victory to shore up her hold on power and thereby go on to have what seems to have been a pretty successful reign.
Counter-intuitively, for such a supposedly overwhelming Russian victory, Russia agreed in the treaty to return Moldavia and Wallachia to the Ottomans–this suggests that, in spite of their victory, the Russians were not especially confident about their ability to hold these places against a later Ottoman effort to retake them. But what the Russians got was so much more important. Through this treaty Russia finally achieved one of its longest-running foreign policy aims: a port on the Black Sea. Actually, they got two, with the promise of more; the Ottomans ceded the port cities of Azov and Kerch to the Russians and, more significantly, were forced to acknowledge the “independence” of the Crimean Tatars. When you phrase it that way, it actually sounds nice, like the war was fought in defense of Crimean liberty or something. But in truth, the Tatars didn’t want to be independent from the Ottoman Empire, and at any rate they never really were; after Küçük Kaynarca they quickly came under Russian domination, and the peninsula was officially annexed by Russia in 1783. Crucially, the two sides also agreed not to interfere with each other’s shipping–which didn’t really do much to benefit the Ottomans, but was important for Russia, because it meant that ships going to and from those new Black Sea ports would have unimpeded passage into the Mediterranean.
But arguably the most important of the treaty’s concessions to Russia involved the right to build an Orthodox church in Constantinople (the first new Orthodox church built in the city since 1453) that would be under Russian protection. This seems innocuous enough, especially since the church in question doesn’t ever actually seem to have been built. However, in years to come Russia would construe this point as giving it jurisdiction over all Orthodox Christians living within the entire empire. Now, this is a really expansive reading of what the treaty actually says, to the extent that you can even figure out what the treaty actually says (it had to be translated into many languages, and the various translations all wind up reading just a little different from one another). But what the treaty said didn’t matter as much as what the Russians, by virtue of their ongoing military dominance over the Ottomans, were able to claim that it said. This greatly enhanced Russian prestige, giving Russian emperors (or empresses, as the case may be) a claim to be the defenders of Orthodox Christianity (which put Russia on par with France, which claimed the same position with respect to the empire’s Catholics). Russia’s assertion of its right to protect Orthodox Christians living within the Ottoman Empire would later help spark the Crimean War.
Küçük Kaynarca, as I say, really made the Eastern Question one of the great preoccupations of European diplomacy. The war itself seemed to demonstrate that the Ottomans were now the weaker party in their endless conflict with Russia, and the treaty’s lopsided terms gave the other European powers some sense of what the world would look like if the empire were to finally pass into history, and the degree to which Russia might benefit (thereby upsetting the European balance of power). The rise of Napoleon helped delay Russian expansionism a bit, since the Russians had bigger fish to fry for much of the early part of the 19th century. But by the time that Crimean War rolled around, the situation was dire enough that Britain and France opted to intervene alongside the Ottomans to prevent its total conquest by Russia.