It seems perhaps worth, in light of that last trope, describing one of those situations in which the Empire leaves people free to pursue their own foreign policies, if they so desire, so let’s skip right to the big one. It is, if you will, an argument in two parts. The former runs like this:
1. It is an established principle of ethics that there is no alchemy which grants to groups greater (or lesser) rights than the sum of those of their constituent individuals. The form of their exercise may change; their essential nature does not.
2. The sovereignty of a polity, therefore, is no more than the collectively-exercised sovereignty (self-ownership) of its citizenship.
3. The right of a polity to make war, therefore, is no more than the collectively-exercised right of an individual to engage in the use of force, and is legitimate for equivalent purposes: namely, the defense of his own/their own sophont rights and/or the sophont rights of others. (Wars carried out for other purposes, as in the case of individual use of force for other purposes, are eo ipso illegitimate.)
4. By simple equivalence, an individual sovereignty (whether or not part of any polity) also possesses “the right to make war” on his own behalf for the defense of his sophont rights, inasmuch as this is ethically equivalent to any other use of force.
Such is merely standard Imperial jurisprudence. Here, though, is where it gets messy:
1. By the principle of consent, as derived from the right of domain (liberty and property) and that of contract, no sophont may be obligated against his will.
2. From this, therefore, we know that every sophont comes into this universe free and unencumbered by any obligation whatsoever, since no individual can bind itself previous to its existence.
3. The legitimacy of a polity’s governance, therefore, rests upon the voluntary (and explicit) assumption of the associated obligations by its citizenship, in accordance with the principle of consent. These polities we refer to as Societies of Consent.
4. It further follows that where such obligations are imposed upon sophonts without their consent, such as by birthright citizenship, descent citizenship, annexation, majoritarianism, or other force majeure, such governance is illegitimate by reason of violating the sophont rights of all its claimed citizens.
5. And as eo ipso illegitimate sophont rights-violaters, such nonconsensual governances are therefore legitimate targets for the exercise of the Right of Common Defense.
Most people, perhaps fortunately for galactic stability, treat this particular piece of ethical reasoning in much the same way as we do our “yay, democracy! boo, dictatorship!” fillip – namely, as something that’s obviously true, and always good for a denunciation at the Conclave, and handy to tack on to whatever other casus belli you might have this week, but it’s not like it means they have to go out and rough up every example of the kind right now, any more than we here in the Western democracies feel the need to go to war with every dictatorship on the planet just because they happen to be one.
On the other hand, it is due to the small number of people who look at said argument and add:
6. (insert plan here)
…that the Empire still manages to produce the odd filibuster . It is, after all, not actually illegal *there* to make war on icky dictatorships and icky democracies and other tyrannical so-and-sos with no respect for the rights of the individual soph, and indeed, it would be a severe violation of the Contract and the Charter were anyone to actually try and make it so . As their delegation to the Conclave of Galactic Polities has regretfully explained on more than one occasion.
 It almost never works, mind. Experience has long since shown them that you can’t liberate people by force when they’re not free inside their heads, and so more practical types stick with subversion and snakeheading when it comes to kicking the slavers where they live. But even advanced, wise, and ultratechnological civilizations are not free of a certain quota of young, idealistic idiots who will convince themselves that surely it will work this time, even if it never has before.
 The very unofficial policy of the Imperial government, Navy, et. al., on this sort of thing is that it’s not illegal, no, but it’s also not official Imperial business and if you’re heading out to start something, you’ve pretty much voluntarily doffed the cloak of Imperial protection. If you deliberately pick a fight with a government and lose, it’s not their job to haul your ass out of the fire either – because you don’t have the right to potentially drag the rest of the citizen-shareholders into your private little war. You go a-conquering, you take your chances, win or lose.