Written in 1919 in the aftermath of World War I and the Russian Revolution, Yeats' "The Second Coming" is about a dramatic shift in history. The first line is a geometric image of one era spinning to its end and the other beginning in its expansion. The gyre, like a cone, spins to one end and a new age emerges. This image describes a reversal in history and thus marks a balance between the past and the future. Yeats entertained the idea that history moved and changed in cycles. In this poem, this particular cycle is 2,000 years: 

The darknes drops again; but now I know

That twenty centuries of stony sleep

Were vexed to nightmare by a rocking cradle, (18-20)

Although the poem's title refers to the second coming of Christ, the indication is more general. That is to say that this "second coming" is just a monumental shift in history. In the wake of the first World War and increasing fascism, the speaker of the poem envisions something more like the anti-Christ ("rough beast . . . Slouches towards Bethlehem) or, more generally and secularly speaking, something like a second "dark ages." 

This moment of great change, juxtaposed to periods of rebirth such as Christ's birth and/or the Renaissance (rebirth), suggests that this rebirth as a "second coming" will be decidedly less wonderful. The poem presents a nightmare scenario for the future. Despite a flourish of Modern art and advances in science (Einstein) in the early 20th century, the world waged one of the most destructive wars in history. The poem suggests that despite such individualistic achievements, the wheel of history marches on. But this is also a contradiction/conflict because people themselves are responsible for the social affairs of the world. This shows the contrary forces at work: the cyclical movement of history and the individual fighting with or against it. 

Christ's second coming is announced by the coming of the Anti-Christ. This suggests that any such moment is marked by the bad and the good. So, even the transitional period itself is marked by a contradiction as if to say a thesis and its antithesis result in a synthesis. The allusion to the "second coming" combines a religious theory of history with a material theory of history. In this case, the poet/speaker does not see the (Christ-like) light at the end of the tunnel. He only sees a dark time approaching.