Yeats writes this poem after World War I. The conflict had left Europe completely ravaged, cut adrift from any hope. The victors were left to survey the immense loss of human life and transformation of their society, while the losers had to deal with this reality as well as the shame of losing. Both sides began to understand that the principles and ideas which guided them into war had been tested, and to a certain extent, deemed false. In this vacuum where disbelief and a lack of faith began to permeate Europe of the 1920s. In this, Yeats writes his poem. The first stanza reveals a world where disorder and disunity reign supreme. The elements that used to define consciousness, such as "the falcon" heeding the words of "the falconer" are absent. The idea of unity and symmetry has been supplanted with a reality where "the center cannot hold." This condition ends with a vision of good being silenced while "the worst are filled with passionate intensity." The second stanza seeks to provide the Christian notion of redemption in the Second Coming of Christ. Yeats inverts this vision with bringing forth a vision where what is coming is not redemption but a monster that proves to be worse than anything imaginable. This creature, arising from the ashes of complete disarray and fragmentation, ends up "slouching towards Bethlehem" and waits to be born. Yeats' vision offered helps to encapsulate two fundamental ideas. The first is that the condition of Europe is one where what was proven to be taken as absolute is no longer valid while the second premise holds that something worse awaits in a predicament where hoping for better is the only thing to which one can cling. It is especially prophetic to place this in the context of Pre Nazi Germany.
The basic idea behind this poem is probably best summed up by the phrase "things fall apart." The speaker in the poem seems to be worrying that the world is getting to be out of control -- the falcon is spiraling around, getting out of control of its owner, the center cannot hold, etc.
Furthermore, the speaker implies that there is nothing coming to save us. The second coming, he says, surely must be coming. But then he goes on to say that it is not. Instead, some monster "slouches towards Bethlehem to be born."
So, what he seems to be saying is that the world is falling apart and there is nothing that is going to save us.
Yeats believed that history was formed into 2,000 year cycles, with the birth of Christ having initiated the cycle he and everyone else was living in at the time of the poem.
The beast slouching toward Bethlehem is the next coming. Notice the beast is slouching toward Bethlehem--the birthplace of Christ according to Christian tradition, of course. Something is coming.
Yeats, like Blake before him, created his own mythology to explain existence. The gyre, etc., are part of his personal mythology.
The particular 'world' in this case was the Irish fight for freedom from England, which Yeats knew was going to be a bloody and unjust one in which many youngsters and older "brothers in arms" would sacrifice their lives for the cause. Also on his mind, and being aired in many newspapers was the strife in Europe which could give rise to another world war. In his poem "The Second Coming" W. B. Yeats presents his mixed feelings - his love for Ireland and its freedom - and yet the scary thought that this freedom might end up as anarchy where "the centre cannot hold" and he and all the other citizens would be "turning and turning in the widening gyre" of a frightening new political force in the world.