Was the Middle East such a mess when the Brits and the French were in control?
Until the beginning of WW1, the Levant (Lebanon/Syria) and Arabia were under the control of the Ottoman Empire. The British wanted control of the area in order to secure their transportation routes to India. Plus, Britain had been waging a cold war with Russia (called the Great Game) for over 150 years along Russia's southern borders. So with WW1 and the Turks (Ottomans) siding with Germany, they (Allenby) kicked them out after making alliances with some locals tribes (the Hashmites who now rule Jordan). But then, France wanted it's own piece of the pie also, since they also won WW1 (they actually threatened war with Britain). So a British Foreign Office bureaucrat named Sykes and the French Foreign Minister Picot made a backroom deal to divvy up the Middle East and create new national borders. That was called the Sykes-Picot agreement. It was that agreement that drew the national borders of the Levant and Arabia that we have to this day. Essentially, the French got Syria and the British got Palestine along a line that stretched to Mosul; the British kept Mosul/Baghdad/Basra because of the oil and created Iraq and installed their own puppet, the Hashemite King Faisal, who got bumped off a few years later RIP. In the meantime, the agreement double-crossed the alliances Capt Lawrence (of Arabia) had arranged with the Bedouin tribes who had helped the British oust the Ottomans; instead of ruling over Palestine from the Mediterranean to the Iraqi desert, they got Transjordan, from what became the pre-1967 war Israeli border.
The French then split what had been the Ottoman province of Damascus in two, reflecting the religious aspect of the population: Majority Christians in Lebanon, and majority Muslims in Syria. The British got a mandate to rule Palestine by the League of Nations, and would later divide it up to accommodate the European Ashkenazi Jews emigrating to Palestine after WW2. Sephardic Jews had lived in Palestine and much of the Arab world for centuries before, BTW.
Long story short, the borders didn't really reflect the ethnic makeup of the populations. It reflected their Imperial economic spheres of influence. Hence the instability. Plus, the Palestinian problem that remains to this day.
Fast forward 8 decades later, and George Dubya Bush decided to upset the balance of power in the area by invading and destroying the Iraqi state of Sunni Saddam Hussein in 2003, partly because Iraq was the only viable threat to Israel, under the pretext that Saddam has WMD's. Later, after they installed 'democracy', it allowed the majority Shiites to gain control of the government, a Muslim sect that Sunni fundamentalists (including the regime in Saudi Arabia) have a historical hate for. Plus, it allowed the Persian Shiites of Iran to become an important regional power and exert major influence in Arabia through its new fellow Shiite friends in Iraq, and its support for the Alawite/Shiite but secular Assad of Syria, as well as the major Shiite faction in Lebanon: Hezbollah.
So the US wants to unseat Assad because he's friends with Hezbollah and both are major foes of Israel; plus, they are allies of Iran and Israel can't stand an Iran that is a rival to them and the road to Teheran goes through Damascus. The Saudi monarchy, along with its Sunni-Wahhabi clerics hate Shiites, and especially Persian Shiites; that created a convergence of goals between Israel and Saudi Arabia, making them closet allies with the US in the middle. Saudi Arabia, along with it's parent monarchies of Qatar and UAE, funded fellow fundamentalist (Salafist) fighters to fight an insurgency against Assad. Al-Qaida got mixed into it (not surprizing since many of its members and its founder Osama Bin Laden, are Saudis), and other extremist groups got into the picture like Jabhat Al-Nusra (There are now reports that Israel is now cooperating with Jabhat Al-Nusra in Syria in order to counter Hezbollah and Iranian measures to defend Assad). Pretty soon, Saudi control of the Syrian Jihadists was lost (the Saudis only know to throw money at a problem) and ISIS was formed with the view of creating a new state based on fundamentalist Muslim practice of the 18th century: a Khaliphate. They rapidly expanded their areas of control, beyond Syria and into Shiite controlled Iraq, and at one point actually threatened Baghdad.
Turkey has it's own goals. It has been facilitating the transit of every kind of rebel and insurgent across its borders with Syria. But their agenda is to control Syria after the ouster of Assad, just like their Ottoman predecessors ruled before WW1. They want to install their own puppet in his place, even if they have to put up with Jihadists hell bent on decapitating every Shiite they can lay their hands on.
So there. Middle East history the way I see it, in a nutshell.