Under what theory of international relations is it legitimate for one country to invade another because they’re upset that other countries bordering it made treaty agreements with another power?
"legitimate" is a loaded word, but the game theory strongly supports one of these options.
“Might makes right”.
Though, to be fair, it's not the most popular theory of legitimacy in international relations.
[1] https://foreignpolicy.com/2009/10/22/clash-of-the-titans/
What do you propose as punishment?
Why is the US or NATO justified in doing the same to Russia then ?
[Edit] Since the algo won't let me post too many comments too fast.
Sanctions/War/whatever . . . the point is that the actions of the United States in Iraq, Afghanistan and Syria are many orders of magnitude worse than anything Russia has done to Crimea or is planning on doing to Ukraine, so any retort saying "We are not going to war, we are applying sanctions" is about as paradoxical as it gets.
If those actions justify sanctions or war, surely the US should be first in line to sanction itself?
[edit] I mean look, call it what you will. The United States having invaded other countries on no pretext is an explanation for why Russia thinks it can get away with this, but it’s not a justification in the moral sense. They are still agents. And sanctions are not war. Threatening war would be saying we’re going to tell our soldiers to kill Russian soldiers if they invade, and we’re not doing that.
Ever stop to wonder what triggered the change in Russia's force posture?
https://www.newscientist.com/article/dn11014-us-anti-missile...
In the early 2000s they were entirely focused on counter-insurgency in the Caucasus. They had restructured their military to be brigade-based, so they could deploy small formations easily. What convinced Russia to re-focus on larger, nation-state threats in their near abroad? There was a really good article on the subject which I can't find now, written when Russia reformed the 1st Guards Tank Army in 2014. That reformation meant they assessed a serious risk of a peer conflict from their Western border, and needed a powerful combat formation optimized for such a fight. Russia has eaten 3 nation-shattering invasions along the Western axis of approach to Moscow (1812, 1915, 1941), where there are few natural geographic/terrain barriers. Do you see how sticking weapon systems in their near abroad that undermine their nuclear MAD deterrence, while expanding your military alliance ever-closer to their territory, might make them paranoid?
The actual security of Russia re: NATO is not really in question here, though you're right that he may not believe this, and we can't be sure that he does. However given the lukewarm international response to his annexation of Crimea, I'm inclined to believe the economic explanation more. He has just decided (probably correctly) that Ukraine is not a country that NATO will go to war to save.
1611 definitely on that list - it's no fun when your capital gets captured and destroyed, and possibly 1242 (the Northern Crusades, although that particular time it wasn't nation-shattering)
Punishment is what Germany, Japan received. Stripped of their military power and made to pay hefty fines. Has the US ever paid any country reparations for their invasions of soveriegn states?