John Boyd was a fighter pilot in the Korean War, an instructor at the US Air Force Fighter Weapons School, and arguably America's greatest military thinker of the 20th Century. His concept of the OODA Loop helped guide the US military during two wars against Saddam Hussein's Iraq. This 4000-word 'long essay' was originally prepared for the War in the Modern World program at King's College London by Daniel Ford, an American journalist and historian. Revised 2014. With illustrations, source notes, bibliography, and a chapter from the author's book, A Vision So Noble.