Yeats published "The Second Coming" in 1920 as part of his larger collection Michael Robartes and the Dancer, which also included Easter, 1916," Yeats' contemplation on the results of the Easter Rebellion that concluded with the phrase "A terrible beauty is born," the reverse of his sentiment about the Second Coming.
From first line to last, Yeats' principal theme in "The Second Coming" is the breakdown of order and the turning of society from potential stability to inevitable instability. Every image in the poem is meant to instill the uneasiness that Yeats himself felt at the end of WWI, which was formally ended in 1919, but seemed to Yeats and others not to conclude the hostilities between the Allies and, principally, Germany and Austria, but to create conditions in which greater evils faced European societies:
Turning and turning in the widening gyre/The falcon cannot hear the falconer;/Things fall apart; the centre cannot hold. . . . (ll.1-3)
Yeats chooses the image of an out of control falcon in part because falconry, a common sport of the upper class in European societies, is well known to his audience and immediately creates an emblem of a society in which normal control is lost: the falcon, which should respond immediately to its controller, the falconer, is spinning upward in an ever-expanding circle, the result of which is that the "centre cannot hold"--the beginning of chaos for this society.
Other images in the opening stanza--"anarchy is loosed;" "blood-dimmed tide loosed;" "the ceremony of innocence is drowned"--dramatically illustrate Yeats' belief, based on his private mythology of how the world alternates between good and evil periods, that the world has entered a period characterized by lack of control and, more important, the advent of evil destructiveness.
Playing upon the positive connotations of The Second Coming, that is, the second coming of Christ and a thousand years of peace, Yeats calms his readers:
Surely some revelation is at hand;/Surely the Second Coming is at hand./The Second Coming! . . . (ll. 9-11)
At this point, the reader hopes that, despite the negative and threatening images of the first stanza, Yeats refers to the traditional Second Coming, which will usher in peace, but he immediately crushes this hope when he refers to
. . . a vast image out of Spiritus Mundi/ [which] Troubles my sight; a waste of desert sand. . . . (ll. 12-13)
Spiritus Mundi is important because it means the spirit of the world or, better yet, the spirit which animates the world, and this spirit "troubles" the poet's mind as he contemplates something, which he cannot yet see clearly, coming out of "a wast of desert sand." These images are, of course, consistent with the terrible images of Stanza 1, but they also point to a much more specific threat to humanity than a "blood-dimmed tide" or anarchy. Something lurks in the desert, and we know that it's not going to be good.
The threat to Yeats' world, and ours, the thing that is the result of the anarchy and bloody tide, finally lurches into sight at the end of Stanza 3, and we finally understand the ultimate perversion of the Second Coming:
And what rough beast, its hour come round at last,/Slouches towards Bethlehem to be born?
Yeats has transmuted the Second Coming of Christ, the most positive of symbols, to the most negative, the advent of rule by a "slouching beast," not born in Bethlehem but moving threateningly toward it in order to convert the birthplace of Christ to the birthplace of the beast--most likely meant to be seen as the Sphinx, an enigmatic monument that guards the pyramids at Giza in Egypt.