Having fought World War I with oil imported from the United States, Britain was determined to avoid relying upon another great power for its energy needs ever again. Even before the Great War had ended, Whitehall implemented a strategy of developing alternative sources of oil under British control. Britain's key supplier would be the Middle East - already a region of vital importance to the British Empire - whose oil potential was still unproven. As it turned out, there was plenty of oil in the Middle East, but Italian hostility after 1935 threatened transit through the Mediterranean. A shortage of tankers ruled out re-routing shipments around Africa, forcing Britain to import oil from US-controlled sources in the Western Hemisphere and depleting its foreign exchange reserves. Even as war loomed in 1939, therefore, Britain's quest for independence from the United States had failed.