Working at Eglin AFB with mathematician Thomas Christie, Boyd developed Energy-Maneuverability (E-M) theory — a revolutionary approach to quantifying aircraft combat capability. Using stolen computer time (Boyd and Christie accessed the base's mainframe without authorization), they created E-M diagrams that plotted an aircraft's specific energy (kinetic + potential energy per unit weight) across the entire flight envelope. The theory proved that the key to aerial combat advantage was not top speed or maximum altitude but the ability to transition between energy states — to gain, maintain, and spend energy more efficiently than the opponent. E-M theory would directly influence the design of the F-15 and F-16 and represented Boyd's first great intellectual leap: from describing combat patterns (Aerial Attack Study) to explaining why they worked (E-M theory).