Yeats wrote this just after the First World War. You do not need his private theory of history: the image of a falcon drifting out of earshot carries the whole thing.
Turning and turning in the widening gyre The falcon cannot hear the falconer; Things fall apart; the centre cannot hold; Mere anarchy is loosed upon the world,
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The falcon wheels in an ever-widening spiral and can no longer hear the falconer. Things fall apart; the centre cannot hold; sheer anarchy is released on the world.
The best lack all conviction, while the worst Are full of passionate intensity.
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The best people have no conviction left, while the worst are full of passionate intensity.
That's the whole thing.
Read it again slowly — it gets better the second time.