In the Bible's Book of Revelation, Christ is prophesied to return to the world to engage in a campaign, called the Battle of Armageddon, against the forces of evil. Yeats uses this symbolic battle to make a comparison with the war that had just been fought in Europe (World War I), which had been thought of as the war to end all wars. The first stanza in particular is filled with the imagery of war and violence. However, according to the poem, the Second Coming had not yet occurred, and therefore that World War I was only a prelude the "real" Battle of Armageddon. The poem doesn't actually endorse the full and literal Biblical prophecy. Yeats appropriates the Battle of Armageddon as a metaphor for the end of social stability in the modern age.
It is impossible to tell that this poem was written in the twentieth century: it could have been written after any war.