This argument, I have heard in real life, sounds A LOT like the US justification for the war in Iraq. It even comes from people who probably opposed the war in Iraq at the outset.

Consider this: what event in Texas would justify the military of Mexico sacking Houston and then making a beeline towards Washington DC? If you answered "nothing Texas could do to its own people could justify that" then good, you probably oppose the Russian Invasion. And we don't have to discuss the details of some idelogues in Crimea.