Attrition, Liam Philpott. After the initial maneuverist attempts of the early war, reality closed in on (most) of those involved in WWI, and the contest of who could create, outfit, and maintain the largest, best forces for the longest began. This process was not wasteful, it was deeply conservative, not just in terms of outlook, but also in practice. If two equal sides constantly exchange casualties, and those exchanges have consistent but small inequalities in their ratios, and if those inequalities have knock-on effects which accelerate the process…
Thus WWI was won, in a slow grind for both sides. On one side, a crawling victory which only gave way to massive gains in the very end stages. On the other, a death by inches, simply on the wrong side of imbalances and exchanges, which no amount of willpower could rectify. The best example we have (because we need it shown to us repeatedly, and we deny it) that the phalanx wins out over the hero.