World War I was modern for two major reasons.
First, this was the first truly industrial war. This was the first war in which the outcome was really determined by industrial output. It was the side that could bring more guns and tanks to the battlefield that won. This is in contrast to previous wars in which tactics and martial spirit were more important.
Second, this was the first war that really recruited whole societies to get involved. Because the war took so many people and because it needed so much materiel, the war efforts depends on the mobilization of the mass of the population. New sources of labor had to be found (women, blacks in the US...) to work in the factories. Governments had to work closely with industry to make sure that enough weapons were produced. Governments had to push hard to get their populations to loan them money.
In these ways, this was the first modern war. It was so big and so mechanized that it demanded that societies' entire energies be devoted to waging the war.