The purpose of the tank was to have an armored vehicle to increase firepower and durability during battle. Tanks changed the war because now it was possible to advance during gunfire without losing as many people as before. Tanks also had moving guns on them that could be used to push enemy infantry back.
The purpose of poison gas was to flush enemies out of their trenches, or to protect your own troops by preventing the enemy's advance. Poison gas changed the war because the usefulness of the trenches was lowered and people could defend their rear with gas in order to focus on the front of an advance. Poison gas also caused many environmental other long lasting effects that would lead it to be banned.
The machine gun was improved during the Great War to have a better firing rate, accuracy, and velocity. This advancement influenced the war by having a way to spray down an area, preventing people from advancing because of the shower of bullets.
Submarines were invented to increase the naval warfare capability through stealthy missiles from underwater. Submarines were incredible at taking out enemy cruiser ships and gaining control of the waters. The Germans allowed unrestricted use of submarines, which meant that they could take down merchant or neutral ships as the captain's order, making them dangerous tools to civilians as well as soldiers.