Poison Gas is a gas used to injure and kill entrenched soldiers. This is an example of modern warfare because you do not have to be close to people to kill them and it also allows you to kill many people in a trench at one time.
The machine gun was a fully automatic gun that shot big bullets. It required four to six operators due to it's massive weight. This changed warfare because you can now take out big crowds with relative ease using the machine gun.
The tank was a response to a stalemate in trench warfare. It is a huge armored vehicle with heavy weapons mounted on it. The tank modernized war because it allowed people to drive the tank and take out large masses of people and trenches.
Submarines were the first underwater vehicles that were able to shoot missiles. Submarines enhanced modern warfare by allowing people to battle underwater and kill more people.