The war in Syria is no longer in the headlines, but that is about to change.
In recent months the Syrian government has taken back all of southern and central Syria from the rebels.
In the east, the Kurdish-led SDF has entered into talks for reconciliation with Damascus. It's widely viewed that the Kurds and Damascus will eventually find common ground.

That leaves just one last break-away region, but it's a big exception.

The Syrian army dropped leaflets over Idlib province on Thursday, urging people in part of the last big swathe of territory still held by rebels to agree to a return of state rule, telling them the seven year war was nearing its end.

Idlib province has been pivotal in this war.
It's fall to al-Qaeda in 2015 caused Russia to enter the war on the side of Damascus.
Every time the Syrian government took back a region - Aleppo, Douma, etc. - the rebels were allowed to evacuate in order to spare the lives of civilians. Those rebels were bused to Idlib. This has changed the character of the province.

An estimated 2.5 to 3.3 million people – at least 1.2 million of whom are internally displaced – are currently crammed into this largely rural region, which only constitutes 3 to 4 percent of Syria. Before 2011, Idlib was home to no more than 750,000 inhabitants. Syria’s northwest, long a hotbed of armed resistance and the heartland of al-Qaeda-linked operations, has become a real-life dumping ground for defeated opposition fighters and their families from elsewhere in the country. At least 70,000 armed men are currently in Idlib and its surrounding areas, according to one estimate, a sizable minority of whom belong to groups formerly or still linked to al-Qaeda.

70,000 heavily armed, fanatical jihadists is nothing to take lightly. The jihadists, now going under the name Hayat Tahrir al-Sham, or HTS, rejected Turkey's demand to disband.
Eventually Damascus will launch an assault on Idlib, and that will cause a humanitarian nightmare.

Having already taken in more than 3.5 million Syrian refugees since 2011, Turkey has firmly shut its border to any more people seeking to flee, so the prospect of conflict in Idlib is truly a nightmare scenario. All other events in Syria would look like drops in the ocean.

HTS is only one power player in Idlib. The other is Turkey, and that's where things get complicated.

As a principal guarantor of the northwestern de-escalation zone, Turkey’s military has also established 12 “observation posts” ringing in opposition territory from western Aleppo south through Idlib, into northern Hama, and back up through western Idlib’s border with Latakia. These posts began as minor lookout points, but have grown to resemble small forward-operating bases, ringed by barbed wire and increasingly by reinforced concrete walls, and housing fleets of armored vehicles and other heavy weaponry.

It's unclear how a regime assault on Idlib can be accomplished without Turkey being involved. To make it even more complicated, HTS and Turkey aren't exactly allies anymore. Recently Turkey has facilitated the assassination of troublesome jihadists, including members of HTS.
Turkey has even used an alarming term.

Turkish President Recep Tayyip Erdogan has called Idlib a “red line” and vowed to defend any attempt to reclaim the province from Turkey’s sphere of influence, raising the risk of a showdown between his country, which is a NATO ally, and Syria, Russia and Iran.

Turkey's annexation of Idlib is not something that Damascus will simply allow.
All of this is happening in the backdrop of a near complete breakdown of relations between Ankara and Washington.

The Trump administration has imposed sanctions on Turkey's interior and justice ministers. Erdogan threatened retaliation and got the support of most of the Turkish opposition. On Wednesday, Stars and Stripes reported that a group of pro-government lawyers in Turkey have filed charges against several U.S. officers at the Incirlik Air Base, accusing them too of ties to terrorist groups. They are demanding all flights leaving the base be temporarily suspended and a search warrant be executed.
...Earlier this year, he threatened American troops with an "Ottoman slap" if the U.S. tried to block Turkey's military incursion into northwest Syria.

Basically there is no telling how this will play out.