He has been watching boats on the Bosphorus strait for two decades; but, until recently, it had been years since Serhat Guvenc had glimpsed a Russian warship. Common in the Cold War era and again during the Balkans conflict, they had become a rare sight on the mighty waterway that transects the ancient city of Istanbul and separates Europe from Asia.

Now, barely a day goes by when the academic and amateur ship-spotter fails to catch sight of a Russian missile cruiser, landing ship or submarine. They goad Turkey by sailing through the heart of its biggest city to supply the conflict in Syria. “It’s like rubbing salt on an open wound,” Mr Guvenc says.

 Turkey and Russia have supported opposing sides in the Syrian conflict since the uprising against President Bashar al-Assad began in 2011. Since November, when Turkey shot down a Russian Sukhoi-24 jet, the relationship has teetered on the brink of all-out war. But thanks to a 1930s treaty, in  peace time foreign states “enjoy the freedom” to send military and commercial ships from the Black Sea down to the Mediterranean. READ MORE