Who Can America Protect?

Greg Scoblete asks:

[W]hat constitutes protecting people? If you spare 10,000 civilians in a Benghazi or Homs only to set in motion a series of events that winds up killing and displacing as many if not more people over a five or ten year period, have you actually done anything that's morally commendable? Do lives become less important if they're lost gradually instead of in one mass slaughter?