Both Guns and Sleepwalkers explain how Europe got from ‘war can’t happen because we’re so interconnected” to a war — but not how it got to a total war. It started as a series of sweeping movements — a classic cavalryman’s war — similar to the Franco-Prussian. But technology had changed. Notably, machine guns and barbed wire (much easier to deploy than an abatis). And at some point, ‘war guilt’ (Germany’s concern about having to pay, and England and France chasing a sunk cost) made it impossible for either side to back down, so things just kept ratcheting upwards, throwing more men into ‘the meatgrinder’ (e.g. Verdun)