Ukraine

The Ukraine war could be ended if Turkey disallowed any Russian ships to go through the bosphorus.

I'm pretty confident that Turkey has treaty obligations to allow civilian ships through. They can close the Turkish Straits to warships, but not civilian traffic.

kagis

https://en.wikipedia.org/wiki/Montreux_Convention_Regarding_the_Regime_of_the_Straits

The (Montreux) Convention regarding the Regime of the Straits,[1][2] often known simply as the Montreux Convention,[3] is an international agreement governing the Bosporus and Dardanelles straits in Turkey. Signed on 20 July 1936 at the Montreux Palace in Switzerland,[4] it went into effect on 9 November 1936, addressing the long running Straits Question over who should control the strategically vital link between the Black and Mediterranean seas.[5]

The Montreux Convention regulates maritime traffic through the Turkish Straits. It guarantees "complete freedom" of passage for all civilian vessels in times of peace. In peacetime, military vessels are limited in number, tonnage and weaponry, with specific provisions governing their mode of entry and duration of stay. If they want to pass through the Strait, warships must provide advance notification to the Turkish authorities, which, in turn, must inform the parties to the convention.[6][7][note 1] In wartime, if Turkey is not involved in the conflict, warships of the nations at war may not pass through the Straits, except when returning to their base.[8] When Turkey is at war, or feels threatened by a war, it may take any decision about the passage of warships as it sees fit. The United States is not a signatory to the Convention.[9]

Turkey was authorised to close the Straits to all foreign warships during a war or when it was threatened by aggression. Also, Turkey was authorised to refuse transit from merchant ships belonging to countries at war with it.

So if Russia kicks off a war with Turkey, then Turkey can close it, but only to Russian ships.

And my guess is that a number of countries wouldn't want that kind of precedent. Okay, say the obligation to permit for civilian traffic goes away. Countries can just use the thing as a lever.

What's going to happen?

Like, first there's the other Black Sea countries. But more-broadly, imagine that, say, Egypt decided to use the Suez Canal as a lever against European countries? Europe would probably invade again and just take it to ensure that they could transit the strait. You'd have a lot of major powers that would just invade and seize bodies of water critical to maritime transit so that it wouldn't be used as a lever against them.

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