I look at whether or not the "fighters" are recognized by a legitimate government, and what their end goal is.
In the case of American rebels during the Revolution, a government was agreed upon, and thier goal was independence from a government that was oppressing them.
In the case of the U.S.S. Cole, bin Laden's fanantics were behind that...hardly a noble cause, and not a real recognized government.
With Beirut, I always understood that the Marines there were a peace keeping force, and that again, terrorists with unpure motives and no recognition took it upon themselves to commit an attack.