For instance, where did Ukraine break a treaty with Russia to justify breaking Russia's binding acknowledging of Ukraine's territorial integrity?
Neither you nor I are experts on international law, but I would suppose Ukraine's initiative (signaled and/or enacted) to destroy all forms of political and economic ties, coupled with "Kto ne prigaet tot Moskal" is enough of an intentional and targeted injury against Russia for Russia to respond with whatever means it can and sees fit.
Just in case you may have had the wrong idea, I will gladly assure you that International Relations aren't a game of checkers, where we exchange equal pieces that stand on equal footing, and spend an equal amount of time doing so. It's chess to say the least, and 3d chess maybe comes closer.
And since Russia violated the Budapest Memorandum
You speak as though international treaties/agreements/laws aren't merely suggestive road signs, but actual rules everyone is required to follow and is punished for violating. They indeed are such rules, but only when applied to specific countries. For other countries they merely remain as suggestive road signs, and nobody punishes them for violating the rules.
There was no fair play in Ukraine, everyone was playing dirty, including Nuland and EU's double-thinkers and double-talkers who eventually ended up double-crossing Yanukovich. Going back to the game of chess mentioned earlier, one cannot follow the rules, if one's opponent never cared for rules to begin with.
And since Russia violated the Budapest Memorandum, why does she not return the nuclear weapons Ukraine handed over to Russia in exchange for Russia's acknowledging of the Ukrainian borders ?
Sure, let's give nukes to a schizophrenic country that already had multiple revolutions throughout the past 3 decades.
You pretend as though the departure of nukes from Ukraine was solely Russia's desire and initiative. But realistically, none of the P5 members wanted Ukraine to have nukes. It was only natural for the nukes to be transferred to Russia as the official legal inheritor of the UNSC and P5 chair.
Sophistry like that one time when you declared Crimea to be Russian because the Communist Russian Soviet Federative Socialist Republic had ceded Crimea to Ukraine, and since the Communists are the baddies, you don't think the Russian Federation as its legal successor needs to stay true to its word, ignoring that the decisively non-Communist Russian Federation in 1994 explicitely acknowledged Crimea as a part of Ukraine?
Bad choices lead to bad consequences. First of all, nobody considers anything Russia did in the 90s to have been anywhere near "far-sighted" or "smart". Secondly, laws and agreements are written on paper, but the livelihoods of human societies are written in blood. That's why there is a clear distinction between legality and legitimacy. For as long as any formal agreement is in its own nature broken, or no longer fits a rapidly changing environment, it either has to be overturned, overwritten, or removed in favor of a legitimate decision and course of actions that represent the needs of human beings. Humans were not created to benefit laws, laws were created to benefit humans. And for as long as Russia's perspective is concerned, Ukraine's ownership of Crimea in the context of the 2014 chaos breached legitimacy, no matter how many Budapest Memorandums or paper-ish Russo-Ukrainian treaties or promises you will try to hide behind.
R2P exists, right to self-determination exists, and so does exist every state's responsibility to act rationally and tread carefully. Incidentally, all these 3 things magically don't apply to Ukraine in your world, which is just the copy/pasta of the world you read about in western press, which in turn is a copy/pasta of the world narrated by western politicians.
Now let's do a school test:
Considering that: Ukraine and Ukrainian nationalists are pissed about Crimea's departure.
Considering that: Ukraine is on all layers of society hostile towards Russia and Russians due to what transpired in 2014 and beyond.
Considering that: Ukraine lives in a perpetual state of financial and political crises, and doesn't have the resources to re-integrate Crimea's economy, citizens, police, legislation, infrastructure back into Ukraine's national framework.
Considering that: The citizens of Crimea would be highly unhappy about the prospects of going back into the embrace of a state that has proven once and once again to be unstable.
Would you genuinely think that Russia handing Crimea back to Ukraine would be a legitimate decision, that comes from a desire to protect and/or improve the lives of Crimeans? The question is rhetorical. Don't worry answering.
To be frank, citizens of Crimea don't give a flying S**t about Budapest. And neither would you, if you were one of those Crimea citizens.
Even UK's Ministry of Defense that enjoys flying LGBT flags on its twitter account, knows that returning Crimea to Ukraine would end badly for everyone in Crimea and for everyone in Ukraine. But UK's goal isn't to return anything to anyone. UK's goal is to throw jabs at things Russia is sensitive about, so to partake in the bigger rivalry that is and has been unfolding between NATO and Russia across multiple continents.
A vote organised after the completion of the annexation, I might add? Crimea is under Russian control by force, and nothing else.
Give me one good reason why Crimeans, who overwhelmingly voted for pro-Russian parties and candidates for many years, who overwhelmingly speak Russian, who have very little historical relevance to Ukraine, and
who have overwhelmingly told your own journalists they want to be with Russia, would somehow magically prefer Ukraine?
Again, rhetorical question. There is no reason. And you know it.