During the early 1960s, Boyd, together with Thomas Christie, a civilian mathematician, created the energy–maneuverability theory, or E-M theory of aerial combat. A legendary by reputation, Boyd was said to have stolen the computer time to do the millions of calculations necessary to prove the theory, though a later audit found that all computer time at the facility was properly billed to recognized projects and that no irregularity could be prosecuted. E-M theory became the world standard for the design of fighter aircraft. At a time when the Air Force's FX project (subsequently the F-15) was floundering, Boyd's deployment orders to Vietnam were canceled and he was brought to the Pentagon to re-do the trade-off studies according to E-M. His work helped save the project from being a costly dud, even though its final product was larger and heavier than he desired. However, cancellation of that tour in Vietnam meant that Boyd would be one of the most important air-to-air combat strategists with no combat kills. He had only flown a few missions in the last months of the Korean War (1950–1953), and all of them as a wingman.