I think the answer is
ARSE
"Riddle from the one-eyed king"
What can that mean other than an arse? If it weren't for this clue in the title, EARS or SEAR or the unordered set {A, E, R, S} would do.
"Complete me one way - if you do this to friends, you must have many enemies."
Add M to get SMEAR, or T to get TASER. If that's how you treat friends, they will soon stop being your friends and may become your enemies.
"Complete me another - if you're tough you'll do this to your enemies."
Add C to get SCARE.
"Yet in a third way - with this you can keep your enemies from you."
Add P to get SPEAR. Adding B to get SABRE might be less successful.
"And in a fourth way - with this you can always defeat your enemies."
Add N to get SNARE.
"What is this word so entwined with foemen?"
I can't get this bit. If it's a crossword-type clue, so entwined with foemen could give AS (synonym of SO) and perhaps RE can be got from foemen somehow, and then we entwine them together.