“ON THE EVE of the first world war a young Winston Churchill switched the Royal Navy from coal to oil. As Daniel Yergin put it in his book“The Prize”, the reliance on doubtful supplies of oil from Persia rather than Welsh coal turned energy security into a question of national strategy. Churchill responded that “safety and certainty in oil lie in variety, and variety alone.””
The Economist (2018) Special Report: The Geopolitics of Energy. A Good, Clean Fight. The Economist. 17 March 2018, pp.11.