Triggered by the assassination of the Austria-Hungarian heir, World War I saw the end of a generation of men who lost their lives in futile charges against fortified positions manned by machinegunners. The fighting started using what were fairly standard tactics and strategy. However, due to the increased firepower from machineguns and repeating rifles combined with increased accuracy and sheer numbers of these weapons, all combatants began to entrench their forces.
This was done on the eastern front as well as the western, but it was never carried out as extensively. The Germans had the best trench system. Often, the Germans had running water and electricity in bunkers down to 60 feet under the surface. These bunkers were extremely resistant to artillery attacks, but it was still possible to suffer from shell-shock. Once a barrage ended, the Germans rushed up and manned their machine guns to repel the inevitable charge of British or French.
On the other side, British commanders wanted their men to keep an offensive spirit and the men weren't allowed to dig in nearly as much. In addition, they had to send out patrols into the No Man's Land between the trenches. Scouting was also carried out with planes. Planes soon began carrying out other missions including shooting down other aircraft, strafing, and bombing. World War I also revived the use of grenades.
However, instead of having units of grenadiers, men known as bombers were trained in the throwing of grenades throughout each unit. Among the other weapons used on a large scale for the first time were the submarine, poison gas and tanks.