Just throwing an idea in, but what would you guys think about some sort of Geneva Convention, that lays out a few rules about Interplanetary warfare? Like no orbital bombardment of earth? No use of biological Weaponry that could exterminate Mankind? No development of "planet-Killers"? (I mean, come on...we dont have another habitable world in the system, so we better be careful with the one we have it)

Then again, some could refuse to sign it, which would lead to some sort of diplomatically backlash!