Blogger Andrew Sullivan writing about Syria today notes:
Syria as we now know it was created by one Brit, Mark Sykes, and one Frenchman, Francois Georges-Picot in 1920. Originally, it included a chunk of Iraq (another non-country), but when oil was discovered there (in Mosul), the Brits wanted and got it.
Similarly Wikipedia's article on Mosul states:
discoveries of oil in the region just before the end of the war (1918), pushed the British government to yet another negotiation with the French; to include the region of Mosul into the southern zone (or the British zone)
Britain and France were allies and equal partners in shaping the post-WW1 settlements, and French interests in the Eastern Mediterranean and Middle East were powerful and historic. Presumably, also, France was just as interested in oil as Britain was.
How was Britain suddenly able to gobble up Mosul simply because oil had been discovered there? Did Britain concede territory to France somewhere else as part of the negotiations?