The First World War showed the vital importance of oil. Use of oil fuelled aircraft, tanks, motor vehicles and especially warships increased greatly during the war. The war made it clear that major powers had to have secure oil supplies. Britain and its allies found themselves in an oil crisis in 1917. It was overcome, with difficulty, and the Allies' greater oil resources, mostly supplied by the USA, contributed to their victory. The situation was, however, been tight and it was not certain that the USA would be willing or able to provide such large quantities in a future conflict. It might not be friendly and there were fears that its oil production would soon peak. These proved to be wrong, but they influenced policy makers, including US ones, at the time. The most obvious place to obtain oil supplies was the Mosul province of the Ottoman Empire. Britain had several reasons to want the League of Nations mandate over Iraq, but oil was the main reason why it wanted Mosul to be part of Iraq. France, Italy and the USA were all also interested in Mosul's oil. The Sykes-Picot Agreement, signed before the need for oil became apparent, had put only about half of Mosul in the British zone. Britain successfully argued at the series of post war peace and inter-Allied conferences that it should have the mandate over an Iraq that included all of Mosul. Britain made several attempts to form a large, British controlled oil company, but it was impossible to create a scheme that suited all parties or that guaranteed that the company would act in the national interest. A realisation that control of oil bearing territory was more important than the nationality of companies allowed the British to give French and US companies a stake in Mosul's oil. This helped to improve relations between Britain and these two countries. The Italians, who had little to offer in return, did not get a stake in Mosul's oil.Oil did not cause the First World War, but the war showed Britain and other major powers that they needed secure oil supplies. As Mosul was the obvious place to obtain them, this quest for oil helped shape the post war Middle East.