Which treaty still controls warship transit to the Black Sea?

The Montreux Convention of 1936, which gives Turkey control of the Bosphorus and Dardanelles and sets rules that are still doing serious work. Merchant ships pass freely in peacetime. Warships of countries without a Black Sea coast are limited in tonnage, in numbers and to a stay of 21 days - so no outside navy can establish a permanent presence there.

The clause that matters most gives Turkey the right to close the straits to belligerent warships in wartime. It was invoked in 2022, which prevented reinforcement of the Russian Black Sea fleet from the Mediterranean and equally prevented other navies entering - a treaty from before the Second World War shaping a current conflict, administered by one country over a channel that at its narrowest is under a kilometre wide.

It is also the reason the Black Sea grain trade is a strategic question rather than a commercial one. Ukraine and Russia between them supply a large share of the world's wheat and sunflower oil, and all of it leaves through that channel - so a dispute at one end of a strait in Istanbul is felt as bread prices in North Africa, which is a fairly direct illustration of what a chokepoint means.