Violence
"The Second Coming" is a response to a world wracked by violence. Yeats wrote the poem 1919, right after the end of World War I, in which 16 million people were killed in a horrifying display of the power of modern technological warfare and of the continuing conflicts that wracked the supposedly modern, civilized world. The poem voices a sense of shock, dismay, and pessimism about the future that many felt after the war. Lines like "blood-dimmed tide" and "mere anarchy is loosed upon the world" both eloquently describe the horrific chaos of war and violence.
Many people felt as if there could never be another war after World War I; it was even called "The War to End All Wars," because people believed that it was so horrible and destructive that people would never allow something like it to happen again. But Yeats foresaw a darker future, which of course came to pass—World War II began a mere 14 years later, and Yeats's second coming took the form of a modernity that endowed humans with increasingly destructive weapons like the atomic bomb, and continued to force people to question how far into the violence the world could descend. This violent history continues, with unimaginable violence continuing on in the Middle East and around the world today.
Prophecy
"The Second Coming" is a deeply ominous poem, full of foreshadowing. Like mythical Greek oracles of old, delivering prophecies in the form of fragmented predictions of the future, it is full of grandiose, dramatic premonitions that do not necessarily make the future much clearer: all it is sure of is that something is going to happen, and the world will never be the same.
Cyclicality
Circles and cycles reoccur persistently throughout the poem. The first line describes all of existence as a "widening gyre," or an expanding spiral. Later, "indignant birds" "reel" or circle through the sky.
Yeats wrote "The Second Coming" during a chaotic time in world history. World War I had just ended, bringing unprecedented violence and chaos to the world. Also, Yeats had just witnessed the bloody Irish War of Independence, which split Ireland into two parts, as well as the Bolshevik Revolution that tore Russia apart. Traditional borders were collapsing, and precarious new ones were being drawn. Yeats believed, as many historians do, that history operates in cycles, with nothing lasting forever and with events always repeating themselves. He also believed that beginnings always lead back to where they began—to endings, to the absence of whatever constituted the circle.
This poem modifies the Christian idea of the "Second Coming" to imply that the world is returning back to how it was before Christianity: without religious morality to guide it, and without an ethical compass to lead it into the future.
Christianity
The poem's overt Christian themes are written into its very title. Yeats himself was not a Christian; he had abandoned the faith in exchange for an interest in occult spirituality, which involved delving into the esoteric mysteries of the universe. His own father, who had grown up as part of the Established Church of Ireland, rejected Christianity at an early age, and Yeats followed suit. But unlike his father, Yeats was deeply invested in spirituality, constantly seeking a philosophy of life, which eventually led him towards occultism and a sort of religious faith in the power of words.
In this poem, Yeats uses Christianity as a stand-in for all order, ethics, and tradition. He borrows this poem's title from the Book of Revelations, which describes Christ's return to earth after the end times as a "second coming" (the first, of course, having been Christ's return after his crucifixion).
Crucially, in the Bible, Christ's return always occurs after a death—of himself, or of the world, in the case of the Book of Revelations. The world is spinning towards a kind of death, Yeats predicts in this poem, but what rises out of the ashes will not be Christ—it will be a mysterious "rough beast."
Meaninglessness
"The Second Coming" is about loss, about change, and about traditional meanings and values coming apart at the seams.
The line "the center cannot hold" basically predicted (and can be used to summarize) modernism and postmodernism, two gigantic literary genres that defined the twentieth century, and both of which—especially postmodernism—fixated on the idea that much of life is meaningless, hollow, without defined order and certainly without the comforting linearity and order that religion and tradition provide. The center cannot hold—there is no more core meaning or logic to be found, and only time will tell what will rise out of the ashes.
