From Wikipedia

Four Years Of Thunder

Episode Three

The first purpose-designed fighter aircraft included the British Vickers F.B.5 - machine gun armament was also fitted to several French types, such as the Morane-Saulnier L and N. Initially the German Air Service lagged behind the Allies in this respect - but this was soon to change dramatically.

In August 1915 the Fokker E.I became operational — this was the first type of aircraft to enter service with a "synchronization gear" (often referred to mistakenly as an "interrupter gear"), which enabled a machine gun to fire through the arc of the propeller without striking its blades. This constituted an important advantage over other contemporary fighter aircraft. This aircraft and its immediate successors - also commonly known as the Eindecker (German for "Monoplane") for the first time supplied an effective equivalent to Allied fighters.

By late 1915 the Germans had achieved air superiority, making Allied access to vital intelligence derived from continual aerial reconnaissance more dangerous to acquire. In particular the essential defencelessness of Allied reconnaissance types was exposed. The first German "ace" pilots - notably Max Immelmann had begun their careers.

The number of actual Allied casualties involved was for various reasons very small compared with the intensive air fighting of 1917/18. The deployment of the eindeckers was less than overwhelming - the new type was issued in ones and twos to existing reconnaissance squadrons - and it was to be nearly a year before the Germans were to follow the British in establishing specialist fighter squadrons. The eindecker was also, in spite of its advanced armament, by no means an outstanding aircraft - being closely based on a pre-war French racer.

Nonetheless - the moral impact of the fact that the Germans were fighting back in the air, and effectively too, created a major scandal in the British press. The ascendency of the eindecker also contributed to the surprise the Germans were able to achieve at the start of the Battle of Verdun - as the French reconnaissance aircraft failed to provide their usual cover of the German positions.

Fortunately for the Allies, two new British fighters were already in production which were a technical match for the Fokker, the F.E.2b and the D.H.2. These were both "pushers" and could fire forwards without gun synchronisation. The F.E.2b reached the front in September 1915, and the D.H.2 in the following February. On the French front, the tiny Nieuport 11, a tractor biplane with a forward firing gun mounted outside the arc of the propeller (on the top wing) also proved more than a match for the German fighter when it entered service in January 1916. With these new types the Allies re-established air superiority in time for the Battle of the Somme, and the "Fokker Scourge" was over.

The Fokker E-III, Airco DH-2, and Nieuport 11 would be the very first in a long line of single seat fighter aircraft used by both sides during the war. Very quickly it became clear the primary role of fighters would be attacking enemy two-seaters, which were becoming increasingly important as sources of reconnaissance and artillery observation, while also escorting and defending friendly two-seaters from enemy fighters. Fighters were also used to attack enemy observation balloons, strafe enemy ground targets, and defend friendly airspace from enemy bombers.