WWI: Modern Warefare
Poisonous gas was very dangerous and caused many casualties. As a result this new weapon could be tactically placed for enemies trying to flank your troops. It was a slow and painful experience the soildiers would face if caught in the gas.
Machine guns were used to suppress fire upon ones enemy.
The machine gun was used primarily to lay down suppressive fire. Most of the time machine guns needed 4-6 men to operate it. Also on average a machine gun could spit out anywhere between 400-600 rounds a minute.
Tanks were mainly used to push threw enemy line or destroy them. A tank is a heavily armed vehicle with strong but not inpenitrable armor. It had a conons and machine guns posted.
Submarines were used to search below the waters either near or past enemy lines. The submarine was able submerge under water for long periods at a time, and when need be could rise to the surface. The sub,arise could tactically hit and destroy ships using their torpedoes.