...the US cannot refuse to be involved. We can however, refuse to arm either side. In doing so, we would be arming our future enemies, no matter which side wins.
What form of US involvement in Syria would not involve selling arms?
FWIW, I think you're dead right about future enemies.
Air support, no-fly-zone, medical aid (supplies, triage doctors, field hospitals), transportation, food, intelligence, money, assistance to refuges, etc....there is much more to waging war than weapons and ammo.
Still I think we would be giving aid and comfort to future enemies. Whichever side wins will likely be our enemy afterwards.