Of the many spectacular views in Istanbul, vistas of the Bosphorus Strait, a 19km waterway that divides Europe from Asia, are perhaps the most beautiful.
Yet unbeknown to the crowds of tourists snapping photos of the huge bridge or the 15-century Rumelian castle, the Bosphorus has recently taken on renewed importance in the theatre of the Syrian war. Over the past several months, the sea route from the Black Sea to the eastern Mediterranean that passes through Istanbul has been crucial to Russia’s widening and deepening support for the government of Syrian president Bashar al-Assad.
Russian warships and naval vessels, often two at a time, have been seen passing through the Straits en route to Syria from Russia's northern Black Sea ports with increasing frequency this year.
"At least 90 per cent of the military supplies, weapons, ammunition, and equipment that Russia supplies to the Syrian Assad regime are shipped from ports on the Black Sea, primarily Sevastopol and Sochi, through the Bosphorus and the Dardanelles, to the two Syrian ports, Tartous and Latakia," said Chris Harmer, a senior naval analyst at Institute for the Study of War in Washington DC.
Everything from tanks hidden by camouflage, to howitzers, armoured personnel carriers and prefabricated buildings are believed to pass through Istanbul. One vessel, the Ropucha class landing ship 158 Caesar Kunikov, has made six voyages to Syria this year, according to reports. Russia's Black Sea fleet is made up of about 45 warships, though 80 more are expected to be added by 2020.
Assad regime
“The Russians use the sea lines of communication (SLOC) between the Black Sea and the Mediterranean to supply the Syrian Assad regime with tanks, artillery, surface-to-surface rockets, machines guns and individual weapons, as well as the spare parts and ammunition required to operate this military equipment,” said Mr Harmer.
Even more immediately troubling for Turkey, which supports Russia's opponents in the Syrian war – a collection of Islamist and other rebel groups – have been sightings of Russian servicemen pictured holding shoulder-mounted rockets and apparently assuming sniper positions on the Bosphorus.
Some observers see the incidents as indirect but clear threats from Moscow following Turkey’s downing of a Russian fighter jet on the Turkish-Syrian border last November that killed one pilot.
Ties between the countries remain frozen as a result of the incident, which Russian president Vladimir Putin called an "enemy act". Mr Putin has also referred to Turkey as being an accomplice of terrorists, and both sides blockaded each other's vessels for a time in December.
Although the Bosphorus Strait, around which the 15 million inhabitants of Istanbul live, has become essential to Russia's war in Syria, there is little Turkey can do to stop the flow of military equipment that has caused so much devastation in Syria. The Turkish Straits that make up the Bosphorus and Dardanelles passage 240km southwest of Istanbul are governed by an agreement that dates back to 1936 – the Montreux convention – in which all nations with boundaries on the Black Sea are free to pass through Istanbul with few restrictions.
Regulatory control
The Montreux convention gives Turkey regulatory control of the Turkish Straits, and states that aircraft carrier vessels are banned from traversing the waterways. However, today, as during the 1930s, Russia’s position as the leading regional power has seen it exploit the treaty, claimed by many countries to be outdated.
His hands tied, last month Turkish president Recep Tayyip Erdogan pleaded for a regional response to Russia's growing dominance over the Black Sea. "It has nearly become a Russian lake. We should perform our duty as we are the countries with access to the Black Sea. If we do not take action, history will not forgive us," he said.
In December, Turkey summoned Russia’s ambassador in Ankara to protest “provocative acts” by warships passaging the Strait.
According to Bosphorus Naval News, a ship spotting website whose operators monitor passing vessels on the Bosphorus daily, 104 Russian warships have used the Strait so far this year, despite Russian claims in March that it would withdraw its forces from Syria. Last year, just 62 made passage during the corresponding period.
The vessels pass within metres of local commuter ferries and fishermen, and cafe-goers on the European and Asian shores of the Bosphorus are in full view of Russian warships.
The route is crucial for Russia and president Putin’s strategic geopolitical plans because Moscow broadly lacks access to the world’s major waterways as a result of the few operational, warm-water naval ports it controls. But now, in large part because of its access to the Turkish Straits, Russia can position itself as a central player in the Middle East.