The head of NATO is calling for calm tonight after Turkey shot down a Russian warplane, killing at least one of two pilots.
It's the first time the North Atlantic alliance has downed a Russian or Soviet plane since the 1950s.
Russian President Vladimir Putin insists the plane was in Syrian airspace. But the Turks released a radar image which, they say, shows the path of the plane as it briefly entered Turkey.
After 10 warnings without a response, a Turkish fighter jet shot the plane down. A video is believed to show its fiery descent, crashing down in northwest Syria.
Russia denies its plane ever left Syrian airspace, and a visibly angry Putin called Turkey's actions a "stab in the back," describing the Turkish as "terrorists' accomplices."
Turkey had already warned Moscow about violating its airspace, and says Russian planes did so twice in October.
A Syrian rebel group posted video which appears to show the bloodied body of one of the Russian pilots. The fate of the other pilot is unknown.
Russia and Turkey are on opposite sides of the Syrian civil war. Turkey -- like its ally the U.S. -- supports the Syrian rebels, while Russia backs the Syrian regime and began launching airstrikes in September.
Moscow claimed to be targeting the Islamic State of Iraq and Syria (ISIS), but the U.S. says it's also hit so-called moderate rebels and that Russia's real goal is to prop up Syria's dictator, Bashar Assad.
There were hopes that Russia could be coaxed into cooperating with the U.S. and its allies in the fight against ISIS. But after this incident, that now looks uncertain.
From the moment Russian war planes began operating out of an air base in Syria, it was an international incident waiting to happen.
To avoid them, a hot line was set up between the Russian defense ministry and the U.S.-run command center for coalition air strikes against ISIS -- but to no avail.
After Putin's angry reaction to the shoot down, the Russian military said a cruiser armed with surface-to-air missiles would be stationed off the Syrian coast ready to shoot down any planes that threaten its aircraft.
President Obama urged both sides to remain calm and described the incident as the consequence of Russia's policy of flying air strikes in support of the Syrian regime.
"I do think that this points to an ongoing problem with the Russian operations in the sense that they are operating very close to a Turkish border and they are going after moderate opposition that are supported not only by Turkey but a wide range of countries," Mr. Obama said.
Turkey is a member of NATO which has spent the past year and a half scrambling jets and deploying troops to counter what is seen as increasingly aggressive Russian behavior.
The sudden appearance of Russian war planes in Syria represented another threat to NATO, this time on its southern flank.
When the U.S. sent planes to turkey in an effort to increase the number of strikes against ISIS, it also sent air-to-air fighters to defend Turkish air space against Russian war planes.
Russia may be causing problems but, in this particular incident, U.S. officials blame Turkey for overreacting to a minor violation of its air space.