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.
You asked about the poem the "Second Coming" by Yeats. Basically, the poem is a statement about the consequences of World War Two. During World War II around 56 million people died. Entire countries were physically dystroyed by fighting, new weapons such as the atomic bomb were created. Many artists, poets etc tried to explain and critique what they saw. More specifically in this poem...Yeats talks about the "Widening Gyre" which is a reference to swirling ocean currents. In other words the war is sweeping across Europe, people cannot escape it. Even countries like England and France that tried to stay out of the war through appeasement were dragged in. Countries such as the USA that tried to remain isolationist during the war were also dragged into the fighting. Yeats says that anarchy is forced upon the world and that innocence has been lost. He is frightened and concerned about the changes he sees. Yeats explains that things are so bad in Europe and the world that "surely the second coming is at hand" This statement references the end of the world as foretold in the Bible. Yeats seems concerned that the damage the war is causing to people and the world is more than we can handle and will eventually be the ruin of civilization.