Prior to World War I, using aircraft for any military purpose other than reconnaissance constituted a war crime. After the outbreak of the Great War, however, the possibilities of air-to-air combat began to emerge. Pilots began to carry weaponry such as pistols and shotguns aloft. In short order, machine guns were mounted, though typically in a rear facing position or a turret, operated by the observer rather than the pilot. Dogfighting with aircraft in these configurations was an awkward and clumsy affair.
Some pilots began mounting machine guns in a coaxial configuration which gave a distinct accuracy advantage, but in the propeller-driven aircraft of the day, they faced the obvious difficulty of firing through the propeller's arc without hitting the prop blades. French pilot Roland Garros attempted to solve this problem by fitting metal plates to the back side of his aircraft's propeller, angled at 45 degrees, intended to deflect the bullets from the blades. While this design was successful in protecting the propeller, the result of the deflected bullets was dangerously unpredictable, jeopardizing the saftey of both pilot and aircraft.
Anthony Fokker, a Dutch aircraft pioneer finally resolved the problem with his innovative interrupter gear. A relatively simple design, the interrupter gear fits a plate to the prop shaft, directly behind the aircraft's propeller. This plate has a cam for every blade on the propeller, directly lined up with the positions of the prop blades. As the propeller and plate spin, these cams raise a push rod each time a blade is in the vertical position. The push rod, connected to the machine gun's trigger mechanism, simply prevents the weapon from firing any time a blade is in front of the gun barrel. This device revolutionized air combat, allowing pilots to concern themselves with piloting and gunnery simultaneously, and giving rise to an age of aces.