Could Turkey block Russia's access to one of the world's most strategic waterways?
That question has arisen with mutual relations in a southward spiral since the Turkish air force downed a Russian warplane near the Syrian border on November 24. It arguably took on added urgency when a Russian sailor brandished a shoulder-mounted missile launcher within firing range of Istanbul in early December, spooking Turkish officials.
The passages in question are the Dardanelles and Bosphorus straits, collectively known as the Turkish Straits.
Fought over for centuries, they link the Black and Mediterranean seas, and are especially critical for Moscow, providing Russia with its sole maritime route from its only year-round, warm-water ports to major international markets. Crucially, the straits lie along the sea route Russia uses to ship supplies to its forces inside Syria, where Moscow launched a major military campaign in late September.
Any eventual steps Ankara might take could be based on an obscure agreement from 1936 called the Montreux Convention. Signed in the eponymous Swiss lakeside resort, it establishes Turkey's control over the Bosphorus and Dardanelles straits and regulates the transit of naval warships. The convention gives Turkey full control over the Turkish Straits and guarantees the free passage of civilian vessels in peacetime.
Crucially, its Article 21 also gives Ankara the right to close the waterways if Turkey is at war or feels threatened by imminent attack.
Tensions are high but, at least so far, not that high.
President Vladimir Putin described Turkey's shooting down of the Russian bomber as a "stab in the back" and has imposed sanctions in retaliation.
Since then, Russian and Turkish vessels -- both military and commercial -- have been involved in at least two near-collisions, and Turkey has accused Moscow of a provocation in the shoulder-mounted missile-launcher incident. Then on December 13, Russian defense officials announced that one of their ships had fired warning shots near a Turkish fishing vessel in the Aegean Sea after what Moscow called "Turkish provocations."
Turkey this month denied it had any plans to revise the Montreux Convention, as the convention allows for every 20 years. But Turkish Foreign Ministry spokesman Tanju Bilgic added on December 8 that "anything could be subject to assessment due to developments."
Nilufer Oral, a professor at Istanbul's Bilgi University, is skeptical that Ankara would invoke Article 21 to limit the Russian navy's use of the passage, saying such a move would be "highly provocative."
In fact, the Russian Foreign Ministry cited the Montreux Convention in denying its naval forces had done anything wrong during the December 4 missile-launcher incident.
"Guarding a ship is a legitimate right of any crew," ministry spokeswoman Maria Zakharova was quoted by Interfax as saying.
In the latest incident, a Russian Black Sea corvette and a coast-guard boat forced a commercial vessel sailing under a Turkish flag to change course after it was thought to have come too near to a Crimean energy firm's boats towing oil rigs.
Just a day earlier, the Russian Defense Ministry had said one of its warships was forced to fire warning shots to avoid a collision with a Turkish ship in the Aegean Sea.
The strategic importance of the Montreux Convention to Moscow was spelled out by Kremlin spokesman Dmitry Peskov in late November.
"The rules of maritime navigation through the Black Sea straits are governed by international law -- by the Montreux Convention -- and we are counting on the inviolability of the norms of freedom of navigation through the Black Sea straits," Peskov said.
As a Black Sea littoral state, Moscow enjoys benefits under the convention that are denied to non-Black Sea states.
For example, Black Sea states can send warships through the straits with few restrictions. Other countries may only send warships into the Turkish Straits and Black Sea for limited periods (21 straight days). Moreover, no vessel from non-Black Sea fleets may weigh more than 15,000 tons, and submarines and aircraft carriers of non-Black Sea states are banned.
Russia has complained in the past that Turkey was failing to uphold the tenets of the convention.
In April 2014, Russian Foreign Minister Sergei Lavrov announced that Moscow had complained to both Ankara and Washington that U.S. warships were frequently remaining beyond the 21 days spelled out in the convention.
In 2008, Russia accused the United States of violating the convention by sending a naval vessel to the Georgian coast during that country's brief war with Russia, a charge that Washington denied, saying the vessel was carrying humanitarian aid.
While Turkey might be unlikely to use the convention to block Russian warships, there are other ways it could try to make things tougher on Russian warships traveling through the Turkish Straits.
"[Turkey] can apply a stricter regime, whereby it would ask for notification by the Russian side about the transit of their warships to the Bosphorus, and for that notification to be given 15 days in advance," Sinan Ulgen, a visiting scholar at Carnegie Europe in Brussels, said.
Russia uses the Turkish Straits to ship the bulk of its supplies to Russian forces inside Syria, where the Kremlin began a bombing campaign in late September to prop up President Bashar al-Assad. Supplies are sent by ship from Russia's Novorossiisk naval base in the Black Sea to ports in Tartus and Latakia in Syria.
But maintaining the status quo on the Montreux Convention makes sense for both Moscow and Ankara, argues Oral.
"This is a convention that is going to be celebrating its 80th anniversary in 2016," Bilgi University's Oral said. "It has really served, I think, the region and the world very well. Russia, of course, benefits from it as well because it limits access of non-Black Sea powers into the Black Sea, which is a very, very critical matter for Russia. For Turkey as well, this is one area where are interests are quite common."