In American English, the phrase is commonly associated with American servicemen in Vietnam, and to a lesser extent, with American veterans of the Vietnam War.
The particular usage of American servicemen varied wildly. While the phrase could be used literally, it was often used in a figurative, and decidedly fatalistic, sense. The meaning was usually something along the lines of, "I cannot put into words what I mean, but this situation/scene/event/dead body/etc contains all the truth necessary to understand precisely what I mean, if you can only see it through the right eyes. I don't know how to express that truth or I do know the right words but it would be too painful for me to actually express them."
The phrase was the most common example of "grunt lingo" and was repeated ad nauseum.