In a war it is often difficult to protect yourself in open ground. One way to do this is to dig trenches. The problem with trenches is that they are fixed in position, making it difficult to move out of them. The most important example of this was during the First World War, when both sides dug over 500km/300mi of trenches across Europe. Because each side had machine guns, it was almost impossible for either side to get out of the trenches and attack the others because they would be killed by the machine guns. That was one reason the war lasted so long. Few modern wars have used trenches.