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...