moment he will open fire, you turn your machine quickly so as to fly at right angles to him. His bullets will generally pass behind you during the maneuver. During World War I, due to the low power of early aircraft, vertical movements were difficult and extended maneuvering led to a loss of energy. Combat tended to degenerate into individual attacks, the classic "dogfights". One specific maneuver that did emerge was the defensive Lufbery, in which several allied aircraft would fly in a circle so that any attackers trying to position against one of the aircraft would fly directly in front of the aircraft behind them. As engines became more powerful, three-dimensional tactics became available to counter the stalemate of the Lufbery, and by WWII it was no longer effective. Development continued through each war, as aircraft and weapon systems became more advanced. Maneuvers such as the "combat spread" were first devised by pilots like Werner Mölders during the Spanish civil war. A simple, non-turning form of the low-Yo-Yo is depicted in John Godfrey's description of his first kill, flying a Republic P-47 Thunderbolt over Europe during World War II; Breathlessly I watched the 109 in between the breaks in the clouds as I dove. At 12,000 feet I leveled off and watched him up ahead. In diving I had picked up speed, and now had hit 550 miles an hour. I was about 500 feet below him and closing fast. Quick now, I've got time. I checked all around, in back and above me, to ensure that no other [Germans] were doing the same to me. My speed was slacking off now, but I still had enough to pick up that extra 500 feet and position myself 200 yards dead astern. The 109 flew as straight as an arrow, with no weaving. As his plane filled my gunsight, I pressed the [trigger]. Much of the modern energy-management techniques, which are used in maneuvers like the Yo-Yos, were only described scientifically after John R. Boyd developed his Energy-Maneuverability theory during the Vietnam War. Even so, as quoted by the U.S Navy Air Training Command, "1) The basics of ACM have not changed since the early days of aviation, and 2) A fighter pilot must maintain constant aggressiveness for success. As the [Red] Baron would say, 'All else is rubbish.'". Training Basic fighter maneuvers (BFM) are used by fighter pilots during