In "A Second Coming" in lines 13-16 the speaker's vision of a happy time is displaced by the vision of a??no
I see no happy time that you refer to. Two thousand years after the first coming, things are seen to be falling apart:
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.
Nothing very happy can be sensed in those lines or any that follow. On the contrary, to me this sounds a lot like the situation we are living in today. Sides are being drawn in the sand; ideologies are lining up and enlisting fanatical forces. A second-coming is at hand, but not one that will bring another Prince of Peace. The words are far too ominous for that:
Surely some revelation is at hand;
Surely the Second Coming is at hand.
The Second Coming! Hardly are those words out
When a vast image out of Spiritus Mundi
Troubles my sight: a waste of desert sand;
A shape with lion body and the head of a man,
A gaze blank and pitiless as the sun,
Is moving its slow thighs, while all about it
Wind shadows of the indignant desert birds.
Something very bad is coming, something dark, dangerous and ferocious.
Make no mistake about it, from beginning to end this poem is a stark prophesy and a warning. You needn't believe in or heed the warning that it proclaims, but there is nothing happy, cheerful or reassuring in any of its lines.
And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?
It's "twenty centuries" later, and this poem suggests that something is coming and it's up to no good.
I think that there can be many interpretations to this particular poem. At the second stanza, I don't necessarily see a "happy time" as much as a potentially hopeful vision. The idea of "Surely some revelation is at hand" would be a faint glimmer of hope that redemption is present and can be recognized. The idea of a revelation that could be there in the midst of stopping the "widening gyre." The reality offered is that this particular poem offers a vision in the future that is not hopeful, but actually menacing. In this sense, the revelation that is present is one that is more ominous and more fearful than anything in terms of positive vision.
The vision of a happy time, a Second Coming (presumably of Jesus) does not last very long. Immediately after the speaker mentions it, he sees a beast emerging from the desert. The beast is emerging from "Spiritus Mundi" which is Latin for "the spirit of the world." So the beast is emerging from the way we people are today.
So this new vision is a horrible one. It is of some terrible beast that is pitiless and menacing. It is rising up and it is going off to be born. It's not completely clear what this means, but is certainly sounds ominous and menacing.