Explain the poem "The Second Coming" in light of World War II.

I have to offer a different answer than the first one
posted.  Yeats does not write this poem about the Second World War.  He writes the poem
in 1920, when the scars of the First World War were still fresh upon the face and soul
of Europe.  Yeats also writes this poem in the Modernist school of thought. 
Essentially, this means that the institutions that helped to define meaning were see as
not being able to do such a thing.  Yeats makes this clear throughout the first stanza
with images that reflect crisis, decline, and a loss of faith.  Where Yeats does pivot
to the start of World War II is in the second stanza.  He articulates a vision that he
sees emerging out of the chaos and ruin of World War I and this vision, one of an
antichrist, is what he sees taking over humanity, “slouching towards Bethlehem.”  While
it’s hard to see Yeats being able to have fully predicted the rise of Hitler and
European Fascism at the time,  the second stanza speaks to this historical reality. 
This makes the poem almost prophetic in its nature.  Where the connection between World
War II and Yeats’ poem can be made is in the idea that while Europe might have believed
the worst to be over in World War I, Yeats saw a desolation that could only serve to
give rise to what would come ahead in the form of World War II.  I don’t think that the
poem is directly about World War II, but rather alludes to it.