Turning and turning in the widening gyre The falcon cannot hear the falconer; (lines 1-2)
The loss of social order has been a gradual process. It’s like when you wandered away from your mom in a crowded place as a small kid, and you thought you could still see her, but at a certain point you realized you were totally lost. The connection between the central authority of a society – be it the aristocracy or a republican government – has become weaker over the centuries. At a certain point, the link was lost completely, like a satellite spinning out of orbit.
The Second Coming! Hardly are those words out When a vast image out of Spiritus Mundi Troubles my sight (lines 11-13)
The Spiritus Mundi exists outside of space and time. It contains all the memories of the collective human past. The psychologist Carl Jung described a similar idea of the "collective unconscious," which may have been an inspiration to Yeats.
twenty centuries of stony sleep (line 19)
The speaker has a deep historical memory. He measures 2,000 years of history as merely one night of a baby’s sleep. Now that’s what we call "taking the long view" of things!
And what rough beast, its hour come round at last, Slouches towards Bethlehem to be born? (lines 21-22)
It’s like the oven bell going off. Ding! "Your Antichrist is ready!" The speaker has a very fatalistic view of the approach of the "rough beast." It hasn’t come because we summoned it, but rather because its hour has arrived. Its appearance is fate, and there’s no way society can avoid it.