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.