Yeats provided a note for his poem "The Second Coming." The first line ends with the word "gyre," and Yeats explained that word with this statement:
"The end of an age, which always receives the revelation of the character of the next age, is represented by the coming of one gyre to its place of greatest expansion and of the other to that of its greatest contraction."
That may seem to provide more confusion than clarification, but it actually does help us understand this poem. Yeats believed in a theory of history based on 2000-year cycles. The cycle he was living in at the time (1919) began with the birth of Jesus Christ. Now that gyre was reaching the widest part of its cycle--think of it as a cone. The next cycle's beginning, the narrow end of the cone, would come from the widest part of the previous cycle. The wide part of one cycle, the end, would take on the characteristics of the coming cycle. The devastation of the Great War, recently experienced by the world, was presumed to be the nature of the coming cycle. The "second coming," then, is not the second coming of Christ as Christians understand it, but the coming of the next gyre or cycle. Thus the "mere anarchy" the world was experiencing in Yeats' day was a portent of an even more dire and dangerous world to come.