The Montreux Convention of 1936 gave Turkey control over key Black Sea straits, an agreement that is still regarded as a major victory for the country’s foreign policy.

However, after Russia’s invasion, Ukraine asked Turkey at the end of February to enforce the 1936 agreement in order to prevent Russian ships from crossing the straits into the Black Sea to attack Ukraine.

“Did your ships sink in the Black Sea?” says a Turkish proverb. When a person is lost in thought, trying to solve a seemingly unsolvable problem, this expression is used. Because Turkey controls access to the Black Sea, that body of water has put Turkey on a geopolitical tightrope since Russia invaded Ukraine and began military operations from those waters.

The straits of Bosporus and Dardanelles were demilitarized after Turkey’s War of Independence ended in 1923 with the Lausanne Treaty. An International Straits Commission was established to oversee access to and from the Black Sea into the Mediterranean.

However, as the political situation in Europe deteriorated prior to World War II, Turkey sought to amend the agreement and negotiated the Montreux Convention in 1936. Turkey’s control of the straits connecting Europe and Asia gives it a unique maritime power to this day.

Montreux is organized as follows: During times of peace, Turkey guarantees all civilian and commercial vessels free passage. Warships from belligerent states can’t use the straits unless they’re returning to home bases in the Black Sea when there’s a war that doesn’t involve Turkey.

Ukraine’s Ambassador to Turkey, Vasyl Bodnar, was quoted in Turkish media in late February as requesting that Turkey implement the Montreux Convention and prevent Russian ships from crossing the straits to attack Ukraine.

When Turkey’s Foreign Minister Mevlut Cavusoglu referred to Russia’s invasion of Ukraine as a “war” in a CNN Turk interview on Feb. 27, he gave Turkey the right to invoke the Montreux Convention. 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.

“However, once these Russian Black Sea fleet ships are in the Black Sea, they will not be able to return to the Mediterranean,” Ulgen said. In the long run, this could pose a problem for Russia’s ability to project power in the eastern Mediterranean, particularly in Syria.”

The sinking of the Russian warship Moskva in the Black Sea on April 14 highlighted Russia’s dilemma: if it wants to replace the Moskva, which was Russia’s Black Sea flagship, or move the Black Sea fleet away from Ukraine, it must persuade Turkey to open the Bosporus and Dardanelles.

There haven’t been any major disagreements between Moscow and Ankara over Ukraine yet. However, there are concerns that their relationship will deteriorate. Turkey is attempting to mediate a peace deal between Russia and Ukraine and has not imposed sanctions on the Russian government.

Turkey, on the other hand, is a NATO member. In a possible sign of escalation between Turkey and Russia, Turkey closed its airspace to Russian planes attempting to fly into Syria on Saturday.

Turkey shot down a Russian jet on its border with Syria in 2015, where Moscow was fighting for Syrian President Bashar al-Assad. Moscow retaliated by prohibiting Turkish food imports and workers from entering the country.

“It’s unclear how long Article 19 will be in effect,” he said, adding that it “was triggered by the Turkish side’s recognition 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.