Ask yourself: if all the military power in the world were controlled by a just ruler, how would it be deployed?
Support deployments of force whenever they coincide with the virtuous plan and oppose deployments of force whenever they fail to coincide with the virtuous plan.
So, for example, if a just ruler in charge of the world’s military power, including the U.S. army, would send in the U.S. army to save Syria, then you must advocate the sending in of the U.S. army to save Syria.
Objection: but you can’t trust the Americans to do the right thing once they’re involved! Reply: according to my rule, you should advocate the sending in of the Americans to do the right thing. If you don’t think they’ll do the right thing (in the sense that they’ll do what a virtuous ruler would do), then you shouldn’t advocate sending them in. My rule is fine with that.
The rule is intended as a refutation of blanket rejectionists of all American military involvement in anything under all circumstances. Suppose that you think that the U.S. government never ever means well toward anyone. But suppose that it looks like, due to incompetence or luck, the U.S. government is about to engage in a military intervention that is precisely what justice calls for. The fact that the U.S. government is behind it shouldn’t matter a lick. You should support the intervention. But only, of course, so long as it fully complies with the virtuous plan.
This might be an argument for the American intervention in Libya in 2011.
You might, of course, conclude that a just ruler would never use military force. But once you pause to consider how the angels of justice would bear arms, it starts to become a lot easier to imagine just conflicts. It seems more likely to me that you might conclude that there are plenty of just deployments of force, but no real military can ever be trusted to execute any of them, inadvertently or not.
Now, the rise of the war machines might make a difference to you, however, because with time one assumes that it will be possible to exert the finest control over their behavior.
Still, whoever is running them will have to want the right behaviors.