It was an enormous, game-changing shock to European mentality; the events of the 30s and 40s would've hardly been possible without it - in any form, Fascism or no Fascism. See e.g. The First World War by Martin Gilbert, a popular history I, personally speaking, liked a huge lot.

As for Britain, it partly wrecked and partly transformed a generation of young men, and dealt a massive psychological blow, from which stemmed the motive for Appearsement twenty years later, and the general slow acceptance that maybe the days of the globe painted red were over. It was time to hunker down, step off the stage, and with good reason; over the next few years, all of Europe began to realize that God would not stop anything man does, or anything that could be done to man.

That latter fact is exactly what Orwell writes about in the essay too, at the point where he quotes "Recessional" to illustrate how Kipling was behind the times in not learning the new age's awful lessons.