The Second Coming -- W. B. Yeats

Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
Following WWI, the earlier Darwinian notion that the world was progressing & becoming a better place was shattered.
The best lack all convictions, while the worst
Are full of passionate intensity.
The Russian Revolution with all its abuses was occurring.

Second Coming = the return of Christ or some other apocalyptic event.

And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?
Doesn't sound like much of a savior will appear, does it?

spriritus mundi Spirit of the world.  Yeats thought there was a universal subconscious in which the human race preserves its past memories.  The psychologist Carl Jung popularized this belief.