I will have to be brief as I am waiting for guests.

Essentially Britain and France had mutual suspicions of each other's intentions in the Middle East since the 19th Century. The 1904 Entente Cordiale papered over these cracks and pushed the issue into dormancy against a common broader strategic interest.

Britain had been happy to see the removal of the Ottomans from Europe well before 1914, however preferred their status quo domination of the Middle East to avoid opportunistic Germans or Russians taking the land to threaten the trade routes to India or access the maritime frontier of India through the Persian Gulf.

The stagnation on the Middle Eastern Front following Gallipoli and the subsequent ruminations in Whitehall that some sort of olive branch to the various Arab groups would be required to alleviate the pronounced shortfall in troops in this second-rung theatre brought this back to the table. Certainly a fear was that a victory for the Entente an opportunistic Russia would loot these possessions (particularly Constantinople and the Straits, an avowed objective for generations) and harm Britain's strategic imperial position.

Furthermore with the planned fall of the Ottoman Empire it was recognised, particularly in the British Foreign Office, that this could reopen the stagnant tensions between Britain and France. This was rightly predicted to be less than ideal during the existential threat of WW1.

Therefore a preemptive 'management' of the situation during the war, as to allow little in the way of opportunities of unsanctioned land grabs to chance after the war was preferential. It was assumed that the old imperial rivalries would open up almost straight after peace so if the situation was agreed while they were allies in such a way and extent to allow little room for subsequent opportunism then Britain's imperial interests would be the most secured.

The first preference for decentralised zones within a continuity Ottoman Empire was considered too risky by late 1915. To this end the comprehensive Sykes-Picot agreement thrashed out the respective sphere of influence and allocated possessions between the big three powers.

A large part of the logic (evident when you look at the possessions) was that Britain hoped for a 'buffer' of French land between Britain and its greatest threat, Russia. The era of the Great Game was certainly not vanquished from the collective memory of the British diplomats so this arrangement would best suit their true asset, India.

Overall the area was already hotly contested and despite being allies in WW1 imperial tensions between the Entente powers were simmering in face against a collective aggressor. The situation had been kept at bay for decades previously with the existence of the Ottoman Empire as a barrier to wandering expansionist eyes between the major powers. However WW1 made clear that this arrangement would likely be scuppered in the event of a Central Powers victory. The Sykes-Picot agreement was an attempt by relatively conservative British officials to avoid Russian and to a much lesser extent French expansionism in the face of a collapsed Ottoman Empire post-victory. It was as much an attempt to protect India and its vital trade routes from the Russians than simply a desire to divide the spoils of the Middle East, though it is important not to understate that this was in itself a nice reward. Overall it is a long-term piece of diplomacy aimed very much at addressing the realpolitik of a future post-Ottoman world while things could be sorted amicably and in a way somewhat controlled by the British.

Sources

Darwin The Empire Project ISBN 9780521317894

Porter The Lions Share ISBN 0582089433