Although hydrogen balloons were used in a communications, scouting, and reconnaissance capacity prior to 1914, the use of air power in a military capacity truly began in World War I.
Britain, France, and Germany each had under 200 aircraft at the start of The Great War. Each nation soon discovered the value of aerial reconnaissance as well as the importance of denying that ability to the enemy. Air Combat was born. The effectiveness of combat aircraft increased dramatically during the early years of the war, and the airplane became a vital force in World War I.
Hydrogen dirigibles also played an important role in World War I. Strategic Bombing had its beginning during World War I when German Zeppelins began raiding London. During the war, Germany conducted over 150 bombing raids on London using Zeppelins. Zeppelins, however, had many drawbacks as bombers. First, was the very flammable hydrogen gas, which provided the lift. Incendiary ammunition proved to be very effective against the airships. Second, was its size. A Zeppelin was over 20 times as long as a bomber, and for all its mass, it was very lightly
armed for air-to-air attacks. Machine guns could only be placed hanging below the massive hydrogen balloon, which gave the zeppelin's guns a very small firing arc. Due to the improvements of Fixed wing bombers in the later years of the war combined with the heavy casualties of vulnerable Zeppelins, the Germans all but discontinued use of the Zeppelin as a bomber by 1918.
Air power also brought the world a new type of hero during the First World War, the flying ace. The most famous by far was Manfred von Richthofen, the "Red Knight of Germany", also known as The Red Baron. Richthofen was credited with shooting down 80 enemy aircraft during the war. He was killed on April 21, 1918.