This sonnet was written by John Keats in the fall of 1819, towards the end of the burst of creative genius that produced most of his best work.
The subject of this poem is of course Fanny Brawne. Her love affair with Keats has been the subject of much speculation, and since his death many critics have accused her of treating him very badly. Quite probably this is not true; she was a very young girl, and he wasn't much older. He was going through a period of depression following the death of his brother, and this, combined with his own insecurities, caused him to be very jealous of Fanny's attentions. He was often away, and when he was, would imagine all sorts of things going on behind his back; his imagination would get the better of him, and he would write to Fanny accusing her very harshly for things she didn't do.
This poem is very reminiscent of "When I have fears that I may cease to be," one of Keats' more well known sonnets, written about a year earlier. The difference between the two works is astounding. The earlier poem calmly and rationally lists the poet's most compelling reasons to live; the tone is awe and wonderment, as though written by a small boy who is looking forward into the entirety of his life, and imagining all that it might be filled with, combined with Keats' ever present fear of death. "I cry your mercy" is frenzied, as though he is struggling to write fast enough to get all of his thoughts down on paper. The meter, surprisingly enough, considering the tone, is very solid; this is Keats at his best.
I cry your mercy--pity--love!--aye, love!
I cry your mercy--pity--love!--aye, love!
Merciful love that tantalizes not,
One-thoughted, never-wandering, guileless love,
Unmask'd, and being seen--without a blot!
O! let me have thee whole,--all-all--be mine!
That shape, that fairness, that sweet minor zest
Of love, your kiss,--those hands, those eyes divine,
That warm, white, lucent, million-pleasured breast,--
Yourself--your soul--in pity give me all,
Withhold no atom's atom or I die,
Or living on perhaps, your wretched thrall,
Forget, in the midst of idle misery,
Life's purposes,--the palate of my mind
Losing its gust, and my ambition blind!