It's intended primarily as an individual guideline, but when you accept that grouping together doesn't grant the group any rights that the individuals comprising it don't have, you see that it works just fine for groups, too.

As for a "preventative strike," a provocation is needed, an actual threat. (No, building a nuke power plant is not provocation) Since groups have no more rights than individuals, we can make an analogy, starting with an interpersonal interaction, and expanding it to interaction among groups.

A man with a pistol on his hip is not threatening anyone. Pulling that pistol out and pointing it at someone, however, is a clear threat. Since no person has the right to initiate the threat of force, it is acceptable to respond to that threat with a preventative strike, to keep him from acting on that threat. Of course, it's prudent to respond to a threat with a threat of your own, proportional force, to prevent any unnecessary violence. If he's informed of the consequences of making good on that threat, he may back down.

To expand that to groups, a group that has weapons, or builds the things they need to make weapons, isn't threatening anyone. If they mobilize those weapons, or place them in such a way as to threaten another group, then they are making a threat. And since no person has the right to initiate the threat of force, neither does a group. Of course, given the greater potential for loss of life should the confrontation get out of hand, proportional force becomes even more important. The resolution of the Cuban Missile Crisis is a good example (even if the events leading up to it are horrible examples). Everyone put their guns away, and the world heaved a sigh of relief.