"The Second Coming" by Yeats presents a second coming of a different kind; different from the second coming Christian tradition presents.
The poem presents social upheaval and nightmarish violence. The falcon cannot hear the falcon, and thus cannot be controlled: chaos results. Those who could do something about the discord "lack conviction," while the worst are filled with "passionate intensity." An ambiguous, sphinx-like creature with a "gaze blank and pitiless as the sun,/Is moving its slow thighs" toward Bethlehem (metaphorically) to be born. The "rough beast" is coming. Nightmarish violence is coming: not the second coming of Christ that tradition expects.
Human history is about to take a turn for the worse.
Yeats is responding to upheaval and violence in the world, and particularly in the Russian revolution. The 20th century is on the verge of something terrible, the poem suggests. And of course, Yeats was correct. Human beings were never so good at killing other human beings as they were in the 20th-century.