WWI Modern Warfare
During World War I, the Germans and the Allies both used several types of poison gas rather effectively. These ranged from chlorine gas early in the war to phosgene gas. Gas was introduced by the German, I.G. Farben and then other nations began to use it. Poison gas made it easier to kill many people at once and people who deployed the gas did not have to be physically there.
The machine gun was improved by World War I and it could easly wipe out troops. It also made it difficult to advance. In theory they could fire 400-600 small-calibre rounds per minute and required a 4-6 man gun crew. This was an efficient attack that did not require a troop of soldiers to take down the enemie's troop.
The tank was introduced by the British at the Battle of Somme in 1916. It was an rough terrain vehicle. The tank ended trench warfare and gave some mobility to the troops. Even though it was unreliable at times. It had a crew of ten men with two machine guns on board and one light artillery gun.
The Germans introduced the submarine in 1914. It's primary weapon was a underwater missile. It caused tremendous damage to ships and were not easily detected under water. Ships were sucked and so were the supplies witch effect enemie's troops.