The Russian fighter-plane incident
Background on the key players:
1) The Syrian civil war pits the Asad regime (dominated by the Alawite sect, an offshoot of Shiite Islam) against several rebel groups including Dâ'ish (ISIS), Jabhat an-Nusrah (an al-Qâ'idah affiliate), the Kurds of the northeast, and various other ethnic and sectarian rebels. Some of these rebel groups, such as Dâ'ish and the Kurds, are also fighting each other. Syria's population is very heterogenous, but the majority is Sunni; militantly Sunni groups such as Dâ'ish and Jabhat an-Nusrah loathe the rule of the "heretic" Alawites. The Asad regime has a history of extreme brutality, which also fuels the rebellion.
2) Turkey is a secular democracy, but its current ruling party is Islamist, authoritarian, and belligerent in foreign policy (imagine a sort of Turkish version of the Republicans). Its military power far outclasses that of most other Middle Eastern countries, thanks to its NATO membership which gives it access to US technology and training. Turkey's grudging support of the fight against Dâ'ish is complicated by its long-standing enmity toward the Kurds -- the huge Kurdish minority in southeastern Turkey has been waging an on-and-off fight for independence for decades, sometimes including terrorism. Like the US, Turkey favors the removal of the Asad regime in Syria.
3) Russia supports the Asad regime (which is a cooperative client of both Russia and Iran) and has recently involved itself in the Syrian civil war to attack not only Dâ'ish but also the more moderate rebel groups, some of which are US-supported. Putin's Duce state has a history of increasingly aggressive behavior which includes challenging the airspace of NATO countries, notably Britain.
In the area of Syria where the plane incident happened, the anti-government rebels are members of the Türkmen ethnic group who, as their name suggests, are related to the Turks and speak a form of Turkish. Turkey tends to be protective of Turkic peoples outside its borders, and has demanded that Russia cease operations against the Türkmen rebels, a demand Russia has ignored (this suggests another possible motive for Turkey's aggressive stance toward Russian forces in the area). The most disturbing part of the story is not the downing of the plane itself, but what happened thereafter.
Both Russian pilots successfully bailed out over Syria before their plane crashed. However, the Türkmen rebels boasted that they shot at both Russians as they parachuted down, killing them in midair. Russia now says that one pilot has been rescued alive, but the Türkmens may well indeed have killed the other in his parachute, which would constitute a war crime. They also fired at a Russian search-and-rescue helicopter which was looking for the pilots, killing one crew member. At the very least, if this rebel group is one of those which has been receiving US support, that support needs to end. And even if Russia is shown to have been in the wrong about violating Turkish airspace, Putin cannot let the cold-blooded murder of a Russian military pilot go unanswered. If Russia escalates operations against the Türkmen rebels and Turkey continues to try to protect them, further clashes are likely.
The US has been publicly supportive of its NATO ally Turkey, but is no doubt working on both sides behind the scenes to de-escalate the situation. Once again we're fortunate to have a leader like Obama at such a time. I don't even want to think about how a President Rubio or, Satan forbid, a President Trump would be handling this.