You see how stupid I am? I never noticed the word 'enemy' at the start of your post. That makes the pic all the hornier, two hot dudes fucking each other while each expecting to kill the other afterwards, man they must be sooo hard!
I don't think they're enemies. They are wearing the same uniform. I think the artist is trying to say that many a serviceman loves his buddy, but never would try to approach him with physical love- even though he may fantasize about it. Also, many a young serviceman is not aware, or certain of his sexuality. That might happen to him in later stages of his military career.