We already have established means of formally legitimising such violations of state sovereignty as we might like to engage in. That's not an immediate practical issue. If we want to invade somewhere, we'll have the UN declare them in violation of some set of accepted principles about how one should run a country or something. (Of course, this is a much harder proposition if, say, we're thinking of deposing a Russian client, but my point is that the initial justification part is a secondary concern. We'll think of something.) Setting up a replacement government that will leave us smelling like roses at the end of the day, though... now that can be very difficult.

(Also, sorry for flooding you like this, but I should also clarify that I'm not taking a position on whether going to war for western ideals is desirable or not. I'm merely identifying a... let's call it an evidently recurring technical problem that seems to be involved with doing so.)