Why did william ernest henley write the poem Invictus?

Henley was admitted to the Edinburgh Infirmary in 1873 following a bout of tuberculosis, and this poem is believed to have been written as a result of his experiences there. He had the option to have his leg amputated, but he chose to have the tarsal bones removed and the joints fused. Henley remained in the hospital for about a year, during which time he began writing poetry. This work was written during his struggles and is considered to be a very powerful poem about perseverance.