War Machine to Pen Pal Fiancee

... 'I Feel in the Marrow of My Bones'

War Machine's got a way with words ... more than enough to make his pen pal-turned-fiancee swoon.

TMZ's obtained a letter the former UFC star wrote to Ashley Farrington ... the woman he proposed to by reading her a poem back in February. As we first reported ... War got down on one knee and asked her to marry him despite having no ring. There's one other thing ... he's serving a life sentence for a brutal attack on his ex, Christy Mack.

War starts his poem by expressing the pain he feels inside. He presses on ... asking Farrington, "Were you once in me? What did God do? Did he cut me open and make out of me, a you?"

As we've reported ... War was found…