Invictus

A friend of mine described herself, after emerging from two weeks of hospital unpleasantness, as being "bloodied but unbowed". I thought, ah, must be another quotation from Shakespeare, but it isn't. Here's the poem, Invictus, from which it is taken, It is a new one on me, as is its author, William Ernest Henley.
Any ideas about the circumstances which caused him to write such a poem?

OUT of the night that covers me,
Black as the Pit from pole to pole,
I thank whatever gods may be
For my unconquerable soul.
In the fell clutch of circumstance	5
I have not winced nor cried aloud.
Under the bludgeonings of chance
My head is bloody, but unbowed.
Beyond this place of wrath and tears
Looms but the Horror of the shade,	10
And yet the menace of the years
Finds, and shall find, me unafraid.
It matters not how strait the gate,
How charged with punishments the scroll,
I am the master of my fate:	15
I am the captain of my soul.