On January 3, 1916, France and the UK created the Sykes-Picot agreement, which in effect partitioned the former provinces of the Ottoman empire in the middle east into French and British spheres of influence.
The agreement, which was signed in 1916, and which was kept top secret, was named after the people who negotiated it, Sir Mark Sykes and Francois Georges-Picot, hence the Sykes-Picot agreement.
Under Sykes-Picot, the Syrian coast and much of modern-day Lebanon went to France; Britain would take direct control over central and southern Mesopotamia, around the Baghdad and Basra provinces and over Palestine which would also would've an international administration
The rest of the territory in question—a huge area including modern-day Syria, Mosul in northern Iraq, and Jordan—would have local Arab chiefs under French supervision in the north and British in the south. Also, Britain and France would retain free passage and trade in the other’s zone of influence.
The problem was that when the UK and France partitioned the former Ottoman territories amongst themselves, and basically created the modern middle east, they did so without ANY consideration for the local populations religious outlooks, ethnic background of national aspirations.
For example, modern day Iraq was made up of three distinctive ottoman provinces: a Sunni province a Shiite province and a Kurdish province. The British just joined all three of them together, without any thought about ethinc or religious differences, backgrounds and tensions.
The result of that was that throughout the history of modern Iraq there were a series of bloody revolts, rebellions, massacres and internal fightings between all three ethinc and distinct religious provinces which resulted in the deaths of hundreds of thousands of people, and which is still affecting Iraq and modern day Iraqis to this day
All the British and the French cared about was their strategic interests and nothing else. Thus local people with different religions, backgrounds and cultures were lumped together in an artificially created "states", whose borders were dictated from Paris and London.
That's how Syria, Jordan, Iraq, Lebanon and Palestine (later Israel/Gaza/West Bank) were created, as a result of petty, greedy squabbling between two decaying colonial empires.
Mark Sykes and Francois Georges-Picot.
No one is suggesting that the middle east was an idyllic area free from conflict and war before the UK and France paritioned it amongst themselves, but during centuries of Ottoman rule it enjoyed, for the most part, long periods of stability and relative peace.
Of course all of that would be gone in the modern middle east, the one that was created by western Europeans.
What we are witnessing today in the Middle East, from Gaza to Baghdad, is the direct result of that agreement, which was signed 104 years ago.
"History of modern middle east.", William Cleveland.
"Kingmakers: The Invention of the Modern Middle East.", Karl E. Meyer and Shareen Blair Brysac