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Key to understanding this poem is the way that the poet uses irony to challenge our expectations. The title, "The Second Coming," leads us to believe that this poem concerns the traditional belief of the Second Coming of Jesus, a time of peace and justice for all humanity. However, the second coming of this poem will, by contrast, be associated with a new era of warfare of savagery, characterised in the following lines:
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned.
The poem ends with an ominously ironic tone as the "rocking cradle" at Bethlehem in which Jesus the baby was born is placed next to the arrival of a "rough beast" that seems to symbolise the advent of the grim epoch that humanity will face in the future.
Thus the poem points towards a horrendous future. It is important to remember that this poem was written in the aftermath of World War I, which was a historical event that shocked so many people. Thematically therefore, Yeats uses this poem to express his distrust in the comforting belief of the Second Coming. For Yeats, looking at history only reveals how, more and more, "the ceremony of innocence is drowned" and mankind is characterised by darkness, warfare and violence, as in the First World War. Looking ahead, Yeats sees no light at the end of the tunnel, but an ever more marked collapse of society.