Syria's darkest chapter

The worst humanitarian crisis of Syria’s brutal civil war is colliding today with what could be the war’s most dangerous geopolitical showdown, after at least 29 Turkish troops were killed in an airstrike.

The big picture: The fighting is taking place in Idlib in northwest Syria, where a ferocious Syrian and Russian offensive has displaced 1 million civilians and infuriated Turkey, which borders the region.

• Turkish President Recep Tayyip Erdoğan has repeatedly warned Bashar al-Assad and his Russian allies to cease the offensive by the end of the month or face a Turkish military response.
• Instead, Erdoğan found himself chairing an emergency meeting tonight following the attack, which reportedly came either from Russian or Syrian forces.
• Turkey is reportedly retaliating against Syrian government targets and conferring with NATO.

Zoom in: Syria's campaign to retake the final rebel strongholds in Idlib — backed by Russian strikes on schools, hospitals and homes — has displaced 1 million people and counting.

• Turkey closed its borders to them, but a senior Turkish official told Reuters tonight that some may cross into Turkey soon. The official added that Turkey will no longer block the 3.6 million Syrian refugees it hosts from reaching Europe.

With nowhere to go, many in Idlib are sleeping in cars, in caves, in sports stadiums or on the street in bitter winter weather. About half are children.

• Most are already internally displaced, having fled from other war-torn cities, Hardin Lang of Refugees International tells Axios.
• The situation in Idlib is on course to eclipse the most severe crises not just of the Syrian War, but of the century so far, Lang says.
• “The No. 1 thing that needs to be done right now is to reach a ceasefire,” he says.

What to watch: Today’s events make escalation more likely.

• Soner Cagaptay of the Washington Institute for Near East Policy tells Axios that Erdoğan’s next move will depend on NATO.
• “Turkey cannot fight Russia on its own,” he says. “If there is no U.S. or European support, Ankara will have to swallow this. But then the conflict will spill over, deeply undermining Turkish-NATO ties (Ankara will say NATO didn’t come to help it).”
• If the U.S. and NATO stand behind Turkey, he says, they could repair relations and sever ties between Erdoğan and Vladimir Putin. But it's unclear what exactly NATO would be willing to offer.

Where things stand: A previous request for U.S. help, including for a Patriot missile system to defend against airstrikes, was rebuffed.

• But following tonight’s attack, Sen. Lindsey Graham called on Trump to establish a no-fly zone "to stop the slaughter and get ahead of a humanitarian crisis,” per Al-Monitor.
• However, Defense Secretary Mark Esper told the House Armed Services Committee Wednesday that there was “no discussion” of the U.S. military “reengaging” in Syria, beyond fighting ISIS.

The bottom line: After nine years, one of the war's most tragic chapters is still being written.