- The Montreux Convention of 1936 gave Turkey control over access to key straits of the Black Sea — an agreement that's considered a big win for the country's foreign policy to this day.
- But the agreement was thrust into the limelight after Russia's invasion, when Ukraine asked Turkey at the end of February to apply the 1936 agreement to stop Russian ships from the crossing the straits into the Black Sea to attack Ukraine.
There's a Turkish saying, "Did your ships sink in the Black Sea?" The expression is asked when a person is lost in thought, trying to resolve a seemingly unsolvable problem.
As it turns out, that's the very body of water that has Turkey on a geopolitical tightrope since Russia invaded Ukraine and began military operations from those waters — because Turkey controls access to the Black Sea.
After Turkey's War of Independence officially ended with the Lausanne Treaty in 1923, the straits of Bosporus and Dardanelles were demilitarized. Access to and from the Black Sea into the Mediterranean was brought under the control of an International Straits Commission.
But with the political situation deteriorating in Europe ahead of World War II, Turkey sought to change the agreement and negotiated the Montreux Convention in 1936. To this day, Turkey's control of the straits between Europe and Asia gives it a unique maritime power.
Here's how Montreux is structured: Turkey guarantees freedom of passage for all civilian and commercial vessels during peacetime. When there's a war that doesn't involve Turkey, warships from the belligerent states can't use the straits — unless they're returning to home bases in the Black Sea.
Turkish media in late February quoted Ukraine's Ambassador to Turkey, Vasyl Bodnar, as requesting that Turkey apply the Montreux Convention and stop Russian ships from the crossing the straits to attack Ukraine.
When Turkey's Foreign Minister Mevlut Cavusoglu called Russia's invasion of Ukraine a "war" during a Feb. 27 interview on CNN Turk, it gave Turkey the right to apply the Montreux Convention in full.
In effect, Turkey's enforcement of Montreux blocks Russia from reinforcing its Black Sea fleet from outside, or from moving warships now in the Black Sea back into the Mediterranean.
"These provisions don't change much of the balance of power in the Black Sea," Sinan Ulgen told CNBC. The former Turkish diplomat is now a visiting scholar at Carnegie Europe.
"However," Ulgen said, "once these Russian ships that belong to the Black Sea fleet are in the Black Sea, they will not be able to return to the Mediterranean. Over the long term, that might pose a problem for Russia's power projection abilities in the eastern Mediterranean and particularly Syria."
The sinking of the Russian warship Moskva in the Black Sea on April 14 highlighted Russia's quandary: Moscow has to convince Turkey to open the Bosporus and Dardanelles if it wants to replace the Moskva — which was Russia's Black Sea flagship — or move the Black Sea fleet away from Ukraine.
Moscow and Ankara haven't yet experienced sharp disagreements with each other over Ukraine. But there are concerns their relationship could become more tense.
Turkey is attempting to broker peace between Russia and Ukraine and hasn't imposed sanctions on Russia.
But Turkey is a member of NATO. And in what may be a sign of escalation between Turkey and Russia, Turkey on Saturday closed its airspace to Russian planes trying to fly into Syria.
In 2015, Turkey shot down a Russian jet on its border with Syria, where Moscow was fighting on behalf of Syrian strongman Bashar al-Assad. Moscow responded with a ban on Turkish food imports and workers.
Ulgen said the Montreux Convention could pit Turkey and Russia against each other again.
"It is unclear for how long the tenure of Article 19 will prevail," he said, adding that it "was triggered by the recognition on the Turkish side that there is a war."
"We could witness a scenario," he said, "where Russia claims that the war is over, but the international community and Turkey not recognize that."
Ulgen said he believes Turkey will continue to comply with the letter of the Montreux Convention, because applying flexibility for one side in the war could create tremendous pressure from the other side.
"Turkey would not want to find itself in that position," he said.