Poisonous Gas was used as a chemical weapon. This was a new invention to kill a lot of people or harm a lot of people. People would blister up, burn your eyes or even choke you to death. It brought new fear of dying in the minds of soldiers which brought shell shock to soldiers.
Machine Guns were used in WWI to suppress the enemy and kill a lot of people at one time. Machine guns allows people to shoot over 200 rounds a minute which, suppresses the enemy or kills a lot of soldiers at one time. This changed warfare forever because now you can't charge an enemy. Now you would have to take cover to avoid being shot.
Submarines were used to hide under the water and strike the enemies boats from a distance with torpedos (under water missile). This affects naval warfare because people fear submarines because they can spend a lot of time underneath water without detections. They can strike at any time.
Tanks were used to move across the battle field with out getting shot. With armor aloud bullets not to penetrate through the tank allowing people to move up. Tanks were able to shoot artillery out of there barrel. With the help of tanks it allowed people to move across the battlefield with out getting shot.