When President Erdogan paid a quick visit to Athens in December, he was almost disconcertingly non-confrontational — but that was so last year. The Turkish president is ringing in the new year with a quiet bang by blocking British royal navy mine hunters and assault boats from entering the Black Sea.  That means less naval back-up for Ukraine, to which Britain donated two vessels. 

The move is an audacious rolling of the dice by Mr. Erdogan, who may be betting that the West is distracted by war in the Middle East now and less focused on Turkish geography. For a country that straddles two continents, though, geography is everything. Ankara behaves with brazen disrespect for Israel, and offers succor to Hamas terrorists, but casts itself in the role of mediator between Russia and Ukraine.

As for the Turkish straits, they are what connect the Black Sea to the Mediterranean, and from there, to the world. No one is stopping ships from entering the Dardanelles, also known as the Gallipoli straits, which are situated at the Aegean end of the Sea of Marmara. The manufactured maritime sclerosis, or MMS, starts at the narrow Bosphorus strait, which of course flows through the center of the Istanbul metropolis. 

While the ground war in Ukraine is now more or less deadlocked, the embattled country has had more success on the naval front, and this despite the fact of its not having much of a navy of its own. With long-range precision guided missiles and maritime drones in its arsenal, Ukraine has scored significant hits on Russia’s Black Sea fleet. 

In the long run, though, if Kyiv wants to get its forces behind Russia’s entrenched defensive lines, it will need to do so with shore operations, in the Crimean peninsula but also pretty much everywhere along those portions of Ukraine’s Black Sea coast now occupied by Russian forces. It is widely accepted that even if air cover can eventually be provided from F16s, it will not be enough. Ukraine’s marines, many recently trained in the United Kingdom, will need back up from the sea to jump in the fight. 

From the looks of things, they might not be getting it. Turkey maintains that it is simply adhering to the Montreux Convention, a treaty in force since 1936 that notably restricts the passage of conflicting parties’ warships through the Turkish straits during times of war. By contrast, “complete freedom” of passage is given to civilian vessels. 

As a measure of how seriously Turkey takes its commitment to the convention, Turkish defense ministry officials on Thursday took the unusual step of publicly refuting allegations in the Turkish press that they had permitted minehunter ships of British provenance from entering Ukrainian waters via the Bosphorus. 

At a press briefing, the pro-government Turkish daily Hürriyet reported, those officials stated, “Since the beginning of the war, Türkiye was the first country to characterize the situation as war and to take de facto measures. … We strongly reject the allegations that would undermine our neutral position in this conflict.”

Which brings us back to Athens. If anyone in the West wonders why Turkey as a member of the North Atlantic Treaty would willingly deprive Ukraine of military assets right now, they might look first at the fraught relationship between Turkey and Greece, another NATO member.

Things may be calm between Athens and Ankara now, especially following Mr. Erdogan’s December charm offensive, but it doesn’t take much for Turkey to stir the pot, whether it’s on illegal immigration, energy rights in the Mediterranean, or divided Cyprus. 

Then, of course, there is the Moscow connection — a comity that springs less from historical affinity than mutual commercial interest. Russian tourists like Turkish beaches. Moscow has already invested $20 billion in a new Turkish nuclear power plant. It’s hardly a surprise, then, that Turkey hasn’t signed on to the reams of sanctions against Russia that Secretary Blinken and some (but not all) Europeans periodically tout as a success.

This plot may yet thicken, in part because in Mr. Putin the Turkish president is dealing with a player whose inscrutability outdoes that of the wiliest Ottoman sultan. Turkey is no longer standing in the way of Sweden’s bid to join NATO. Yet as the northern flank against Russia strengthens, on the southern one it’s a different story.  If Mr. Erdogan thinks he can outfox Mr. Putin, he might be mistaken.

The entire Middle East is in a dangerous state of flux, but if pressure can be brought on a bunch of Hezbollah terrorists to pull back from the Israeli border, chances are good the British or Americans can in time dangle a few carrots to get more ships through the fabled Turkish straits. 

The Bosphorus is key. Whoever controls it can open or close the door to the Mediterranean as he sees fit — so who or what would stop the Russian president from eventually making a play for it himself? For Mr. Putin that could be the next “special military operation,” hobbled fleet or not,  on his wish list. For Turkey and everybody else in the neighborhood, it would mean war.