Three years ago, U.S. President Barack Obama drew what he said was his "red line" – the use of chemical weapons – regarding the war in Syria, then backed away from enforcing it.
On Thursday – as pressure mounted for the international community to do something to ease the al-Assad regime's devastating siege of eastern Aleppo – Russian President Vladimir Putin drew his own red line: Russia's military warned that any U.S. air strikes in the country would be considered a threat to the Russian servicemen based there, and that sophisticated anti-aircraft weapons would be used in response.
In other words, the Kremlin owns Syria's airspace and won't let U.S. and allied planes stray from their United Nations-approved mission of targeting the so-called Islamic State. It's a threat backed by S-300 and S-400 anti-aircraft missile systems deployed around Russia's military base in the northwest of the country, as well as Russian warplanes that control Syria's airspace, carrying out daily sorties in support of President Bashar al-Assad's army.
On Friday, the Mirazh, a cruise-missile equipped corvette, will cross through Turkey's Dardanelles strait on its way from the Black Sea to the eastern Mediterranean, where it will join the Russian fleet off the coast of Syria. The Mirazh is the third missile-equipped Russian warship to pass through the Dardanelles this week alone.
The buildup comes as the U.S. administration, which this week ended negotiations with Russia aimed at finding a diplomatic solution in Syria, has hinted that military options – rejected in 2013 – were back on the table as the White House seeks a way to force Mr. al-Assad to end the siege of Aleppo.
"Together, the Syrian regime and Russia seem to have rejected diplomacy," U.S. Secretary of State John Kerry said this week.
Deputy Secretary of State Antony Blinken told the Senate foreign relations committee this week that Mr. Obama had asked staff for "new options" in dealing with Syria.
Russia's response came at a press briefing in Moscow on Thursday. "Any missile or air strikes on the territory under control of the Syrian government, will create an obvious threat for Russian military," said Major-General Igor Konashenkov, a spokesman for the Defence Ministry.
He referred to a Sept. 17 bombing by the U.S.-led coalition that killed more than 60 Syrian soldiers, a bombing that the United States says was a mission targeting Islamic State that had gone awry.
"We have taken all necessary measures to rule out any such 'mistakes' against Russian military and military facilities in Syria," Gen. Konashenkov said. "The crews [of the anti-aircraft systems] will hardly have the time to calculate the missiles' flight path or try to find out their nationality."
The naked threat to shoot down U.S. warplanes was the Kremlin making it clear just how thoroughly he has has outplayed the White House in Syria, a conflict that now seems likely to end only on Mr. Putin's terms.
After all, it was Mr. Putin who led Mr. Obama away from his red line in 2013, drafting a plan that called for Mr. al-Assad to give up his chemical weapons, but allowing him to carry on his war against scattered rebels – at the time largely a collection of U.S.-backed Syrian army defectors – by any other means necessary, including crude "barrel bombs" and the relentless siege of entire cities.
Three years later, the United States has completely lost the initiative in the country. The rebels it backed in 2013 are now a peripheral force, confined mostly to the city of Daraa in the south and the surrounded pocket in eastern Aleppo. The picture has been muddied by on-the-ground alliances between those "moderate" rebels and Jabhat Fateh al-Sham, a group with ties to al-Qaeda.
Meanwhile, the Islamic State has seized much of the east of the country, while Mr. al-Assad's regime – backed by the Russian air force, as well as Shia militias from Lebanon, Iraq and Iran – is far stronger than it was three years ago.
The battle for Aleppo, which before the war was Syria's largest city with a population of just over two million, is considered critical to any eventual resolution of the five-year-old conflict, which has already taken more than 400,000 lives and forced millions to flee their homes.
Mr. Kerry said this week that Syria and Russia had "rejected diplomacy and seem to have chosen instead to continue their pursuit of a military victory over the broken bodies, bombed-out hospitals and traumatized children of a long-suffering land."
On Thursday, Staffan de Mistura, the UN's special envoy for Syria, warned that the world should not look away from what was happening in Aleppo, where 275,000 people have been trapped for more than a month with extremely limited food and medicine.
"The bottom line is, in a maximum of two months, two and a half months, the city of eastern Aleppo at this rate may be totally destroyed," he told a news conference in Geneva. "Thousands of Syrian civilians, not terrorists, will be killed and many of them wounded."
He warned that the world would be watching "another Srebrenica, another Rwanda," unless action is taken to end the siege.