due to the massively increased lethality of the gun, via it's increased range, accuracy, rate of fire, etc, it became nescary for the individual solider to protect himself. the first trenches were little more than ditches, however that the blended in to the surronding made them more surviable than large fortifications, see the forts of Verdun. they were relatively quick to setup, and provide an excellent position for defense, as attacking infantry had to expose themselves to take it, see any offensive in the west, whilst it's defenders were sheltered from the worst offensive fire. the weakness of a trench was it's static nature which allowed it to be outflanked, see the eastern front/ palestine however due to the concentration of forces on the western front both sides were able to fully ma