The fall of Mosul to ISIS, which controls territory in both Syria and Iraq, will probably engender another round of op-eds with titles like "The End of Sykes-Picot," which many mean as shorthand for "the end of the post-World War I border settlement in the Middle East." There are many problems with the phrase, not least of which is that the 1916 Sykes-Picot Agreement gave Mosul to France.
Or, a bit more precisely, it included Mosul in Area "A," the area under indirect French protection as opposed to direct rule. In fact, the fate of Mosul would be disputed continuously well into the 1920s. Mosul's fate was not settled by the Sykes-Picot Agreement of 1916, nor by the Treaty of Sèvres of 1920, nor by the San Remo conference that year, nor even by the Treaty of Lausanne of 1923, which resolved most other issues. It took an investigating commission from the League of Nations, and was only resolved after a Frontier Treaty between Turkey, Britain, and the Kingdom of Iraq in 1926, under which Britain promised to provide Turkey with 10% of Mosul's oil revenues for 25 years. Even after 1926, Turkish leaders have continued to occasionally threaten to revive the claim.
Why was Mosul such a thorny issue? Some background: Under the Ottomans, the Vilayet of Mosul consisted of the Sanjaks of Mosul, Kirkuk, and Suleimaniyya, roughly coinciding with the four northernmost provinces of Iraq today. Then as now, it was a multi-ethnic region, with Kurds, Arabs, Turkmen, Armenians, and several Christian, Yazidi, and until the 1950s Jewish populations. It was far from exclusively Arab, the Turks argued; Arabs noted that it was not overwhelmingly Turkish; Kurdish nationalists saw it as part of an independent Kurdistan.
Oh, did I mention the oil yet? The oilfields around Kirkuk were seen by the British as critical to the survival of the fledgling Kingdom of Iraq. Although the Kirkuk fields did not enter production until the 1920s, British oil interests and the Armenian entrepreneur Calouste Gulbenkian had secured oil concessions in the region even before the war.
Finally, the Turks were determined to recover Mosul because of the oil, of course, but also because the British had occupied it after the armistice.
On October 30, 1918, at Mudros on the island of Lemnos, Great Britain and the Ottoman Empire signed the Mudros Armistice. It called for an end to hostilities on both sides. On November 2, three days later, Lieutenant General Sir William Marshall, British Commander in Mesopotamia, ordered British troops into the Vilayet of Mosul to secure the oilfields. Both the Ottomans and later the Turkish Republic saw this as a violation of the Armistice, and the treaties of Sèvres and Lausanne left the control of Mosul unresolved, though at Lausanne Turkey accepted League of Nations Arbitration.
Meanwhile, as I noted above, the Sykes-Picot Agreement had included Mosul in the French zone. This problem was easier to deal with than the Turkish claim. During the Paris peace talks, on Sunday, December 1, 1918 during a meeting at the French Embassy in London, by David Lloyd George's own account, Georges Clemenceau asked him what he wanted, and Lloyd George immediately replied, "Mosul." Clemenceau then said "You shall have it. Anything else?" To which Lloyd George responded "Palestine from Dan to Beersheba," (or in another version, "Jerusalem.") (Much of Palestine was supposd to be under international control under Sykes-Picot.) Clemenceau, who wanted British support for French claims in the Rhineland, quickly agreed. Even Lloyd George seemed surprised it had been that easy, and French diplomats reportedly were annoyed that Clemenceau had given so much away for nothing concrete in return.
As I have noted, Mosul remained unsettled until the League of Nations, with Turkey's agreement at Lausanne, sent an investigative commission. That in turn awarded Mosul to Britain and Iraq, which Turkey initially rejected. Only in 1926 was the Frontier Treaty signed.