Answer:
Airplanes and submarines
Explanation:
Two new weapon systems that were developed in the early 20th century were airplanes and submarines. Aircraft was first used in war in 1911, but it became widespread during the Great War (1914-18). Airplanes were first used for reconnaissance, and later as fighter planes. Submarine warfare also became important during WWI, mainly as a fight between German and Austro-Hungarian U-boats.
