Fitzgerald's thesis in this passage is that the western powers' prosperity in the 1890s and 1910s - and, more broadly, the social capital built by their national identies across the entire 19th century - created a generation "tough" enough to endure their unspeakable mutual slaughter on the western front.

If he is correct, then it refutes the proposition that prosperous societies are "soft".

Maybe you should do some studying to improve your lateral thinking, because it is extremely obvious.