"Mesopotamia . . . yes . . . oil . . . irrigation . . . we must have Mesopotamia; Palestine . . . yes . . . the Holy Land . . . Zionism . . . we must have Palestine; Syria . . . h'm . . . what is there in Syria? Let the French have that." — Lloyd George during the 1919 Peace Conference, thinking aloud in the presence of Arnold Toynbee [Margaret MacMillan, Paris 1919, page 381]
What is there is Syria? Not Lebanon or Palestine. Both were traditionally part of Syria, but were partitioned off by the Great Powers (the old French Empire and British Empire) in the early 20th century.
How many people are left in Syria? Maybe 18 million. Note that Greece has a smaller population, about 11 million. So why are we so worried about a debt default in Greece, when the Apocalypse has already come to Syria? Because the Greeks owe the rest of Europe so much money. Syrians made the mistake of not borrowing vast sums of money from America and Europe.
Of course, to borrow vast sums of money from Europe and America, the Syrians would have had to give up much of their autonomy. They would have to undertake a pro-American and pro-Israeli foreign policy. And they would have to use the loans to make profits for the lenders, which was a little hitch the Greeks forgot about.
After the French Empire finished mismanaging Syria in 1945, it was left to the locals to mismanage their country. The French apparently trained the locals well.
At one point, when the two empires were dividing up what bits of the world they had not grabbed already before World War I, the city of Mosul and the area around it was supposed to be part of Syria. Mosul was already known to have oil. The French and British argued. They decided Mosul would be part of British Iraq, but that French oil companies would share the profits from Mosul oil, and would allow a pipeline to be built through Syria so the oil could be picked up on the Mediterranean coast.
Syria may not have much, but people want it all the same. It was building up a respectable tourist industry before the current civil war began. So of course President Bashar Assad and the political and economic elite wanted to hold on. It's not like they were in the position to grab anything else. They could not even grab the Golan Heights back from Israel.
Every system, no matter how democratic or how authoritarian, has its outsiders who would like to become a new part of the old elite, or a new elite. Assad and friends had made enemies, and for all I know some of their complaints might have been justified. In any case protest turned to civil war in 2011.
Civil war is usually a bad choice. Maybe the legally elected regime could have compromised more and averted the war, but I doubt it. The Syrian civil war was always about Islamic radicalism. Sunni Arabs account for about 60% of the population, yet Assad is Shia and the government is dominated by the Shia minority. Politically the government is dominated by the Baath party and its allies in the National Progressive Front, which includes many Sunni Arabs.
Some people just never modernize. They think religious platitudes are actually fact-based. They don't want to tolerate anyone who does not believe in the same fairy-tales as themselves. Apparently there were plenty of these people in Syria, because they flocked to the rebellion. Then the I-am-more Islamic than you game got some momentum. Al Qaeda became prominent, then its even-more-radical offshoot the Islamic State.
Syria may not have anything the British Empire wanted after World War I, but when you have nothing, even Syria may be attractive. The Islamic State wants Syria, and since the U.S. has an irrational hatred of Assad and the Baath Party, it won't be a big surprise if the Islamic State gets what it wants.
Wither then? Turkey? More likely Jordan. The difference is that Jordan has long been a U.S. client state. Whoever is President (of the U.S.) at that point will probably try to get Saudi Arabia or Israel to defend Jordan. But likely at that point, if they are not needed somewhere else in the world, U.S. troops will be deployed in earnest.
Even when you are better armed and better trained, fighting fanatics is not a job any sane person wants. They just keep coming at you. You begin to feel like you are a minor character in the Walking Dead series. You can't let your guard down for a moment. Ask any Vietnam Veteran.
It kind of makes me wish we had just let the old U.S.S.R. have Afghanistan and a free hand to convert the lot of them to atheism.