Publicly Hitler proclaimed "We have no disputes with anyone, leave us alone and we will leave you alone." (heavily paraphrased). But diplomatically, it was one thing after another -- oh, its unfair we can't have troops in the Rhineland, oh the ethnic Germans in the Sudeten are being mistreated. Poland can't protect its minorities and they are being harmed. (again heavily paraphrased) Somehow, some way, there was always an issue with a territory that he just happened to want for lebensraum for the German people. Funny how that happened. It was all self-defense so it wasn't aggressive war.
Fast forward to 2014. Putin does the same thing with regards to Crimea. Oh we just want to be everyone's friends (the paraphrasing should be presumed at this point), we have no designs other than peace and love and friendship with everyone. Except, oh the Ukrainian president who decided against closer ties with the EU is facing a revolt, well we have to protect the ethnic Russians there. Said, Ukrainian president fleeing the country and resurfacing in the Russian city of Rostov on Don is just a mere coincidence. He could have gone anywhere in the world, the dart on the map just happened to hit Rostov on Don. Oh hey cool, the plebiscite we happened to run and wouldn't let anyone observe just happened to come back that the folks in Crimea want to be part of Russia again. If the Ukrainians would just stop fighting in the Donbas, we could have peace. I mean its their fault for trying to hang on to territory full of people we have put in there that amazingly enough claim they too want to be part of Russia. Well, Ukraine won't listen when we raise our entirely made up, I mean, valid concerns about ethnic Russians, guess we have no choice but to invade. Really Article 51 says we can act in self-defense.
Except Article 51 of the UN Charter specifically states "if attacked." No one attacked Russian territory. It's not self-defense to invade a country who won't give up territory you claimed. Furthermore, Article 51 is triggered only if the Security Council is blocking action, the other countries may unite to help the attacked country. The coalition helping Ukraine is most likely authorized under Article 51, but Russia's 2022 invasion of Ukraine is most likely not.
Why does this matter? Aggressive War is outlawed, by Kellogg-Briand and by the UN Charter itself. In very few cases can countries go to war without UN sanction. One must be justified in starting a war. There are only a few ways to justify starting a war (finishing it always allowed, especially if it means kicking the butt of the one who started it without justification). But these justifications must be limited in scope. If they are read too broadly, almost anyone could find a justification for war. Hence all the pre-texts. Really mom, HE started it, not me. Sometimes, the country is just reacting to provocation by another country. England declaring war on Germany after the Polish invasion may fall into this category. Germany was warned, don't invade Poland but it did. But most of the times, the aggressor is just looking for an excuse to do what they wanted to do any way.
Hitler knew this. That's why diplomatically he always laid out a good case as to why he was acting. But the Nuremberg Tribunal laid out that many plans for invasion were set in motion long before he raised the diplomatic issue. In more than one memo it was stated "Just waiting for the right opportunity to exploit."
Putin is no different. It's pretty well known that he wants to reconstitute the Russian empire. Ukraine is just one piece of it. But he also knows Russian aggression will not seen favorably even by allies. Thus, the fig leaves of protection of ethnic Russians and "de-Nazification" of Ukrainian leadership. He had the audacity in February 2022 to actually invoke World War II to justify the invasion he launched mere hours later.
Fortunately, this time the world is ready. Rather than wait for the outcome of the war, the ICC has already issued an arrest warrant against Putin for his war crimes. More importantly, the EU Commission has opened a center for member countries to work together to gather in one place all the evidence for his crimes against peace. This evidence will be ready and waiting should any trial come to pass.
Sources:United States Office of Chief of Counsel for the Prosecution of Axis Criminality et al., Nazi Conspiracy and Aggression.Al Jazeera Staff, “‘No Other Option’: Excerpts of Putin’s Speech Declaring War.”International Criminal Court, “Situation in Ukraine.”Brierly, The Law of Nations; an Introduction of the International Law of Peace.Directorate-General for Neighbourhood and Enlargement Negotiations, “Ukraine: International Centre for the Prosecution of Russia’s Crime of Aggression against Ukraine Starts Operations Today.”Walker, Nibel, “Conflict in Ukraine: A Timeline (2014 - Eve of 2022 Invasion).”