"The Second Coming," published in 1919 just after the end of the First World War (though the Allies and Axis powers were still negotiating the terms of Germany's surrender) is one of Yeats' most negative visions of the future. Yeats is, in part, reacting to the tremendous devastation caused by WWI in England, Ireland, and the Continent, and expresses his fear that one civilization has passed away only to be replaced by a new and very dangerous one.
The first stanza includes several images, one unique to Yeats, of a loss of control:
Turning and turning in the widening gyre/The falcon cannot hear the falconer;/Things fall apart; the centre cannot hold;/ . . . The ceremony of innocence is drowned; . . . .
When Yeats uses the word "gyre," he's not only referring to the upward spiral of the falcon (which is no longer under the falconer's control) but also the spiraling out of control of civilization. Yeats uses the image of a gyre--essentially, two spiral cones, each spiraling in opposite directions--to represent the duality of mankind and historical movements, both good and bad. In "The Second Coming," the negative side of the gyre is taking control.
In the second stanza Yeats makes explicit--through Biblical imagery--the the Second Coming is at hand. In a Christian context, the Second Coming refers to Christ's Second Coming and the beginning of universal peace on earth, but as soon as the poet employs this Christian positive image, he becomes troubled by the vision of another kind of second coming:
A shape with lion body and the head of a man,/A gaze blank and pitiless as the sun,/Is moving its slow thighs. . . .
The image is, of course, the Sphinx, and its most important aspect--a pitiless gaze--foretells that mankind is not going to be redeemed by the Christian Second Coming but cursed by something monstrous and "pitiless."
In the poet's view, the Sphinx, after "twenty centuries of stony sleep," was awakened by the unbelievable devastation created by WWI and, more important, by mankind's inability to regain its morality. Yeats' question, then, "And what rough beast, its hour come round at last,/Slouches towards Bethlehem to be born?" brings the original reference to the Second Coming to a shattering conclusion. Rather than the Prince of Peace arriving to bring peace to mankind, we have a perversion of the Second Coming--one in which "darkness drops again."