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Guns: WW1 proved to be an important war in terms of the advancements made in military technology. Guns in particular became more than just a tool for hunting and killing, but suppressing choke points and reinforcing important vantage points.
Aircraft: Like guns, a lot was discovered in the designing of aerial vehicles. The main focus in planes in WW1 was recon, bombing, and dog fighting. What was learned from planes and aerodynamics in WW1 through trial and error allowed there to be much more advanced aircraft today.
Poison Gas: Probably the most cruel of them all, Poison Gas saw most of it's use in the trenches of WW1. The Poisoned gas, usually made up of chlorine and other chemicals, was inhaled by the enemy and caused a significant amount of pain. Some gasses used in the war would cause the victim to become asphyxiated and cough up blood from their lungs, the sensation of poison gas inside of the lungs was like that of a fierce fire burning inside of you.
U-Boats: U-Boats were mostly used for landing operations. These boats would usually but not always be discharged from a larger boat carrying squads of soldiers to storm the beach that they were headed to. These would later be used in bigger invasions like those of the Pacific Theatre in WW2 and most notably D-DAY.