You see, current states have territories and borders that are enshrined in relevant treaties with neighboring states, and are also recognized by all other states of the world by concluding diplomatic, consular and other agreements with them. All of them provide for the sovereignty and inviolability of existing borders, regardless of how these borders have changed over previous centuries. This is necessary in order to stop the endless wars for new territories that have almost always accompanied humanity. According to the UN Charter and other international agreements and world practice, unauthorized changes to existing borders are categorically prohibited and are recognized as territorial aggression with the imposition on such a state of the obligation to restore the previous territorial position and compensate the state that has become a victim of aggression for losses caused to it. This distinguishes the current civilization from previous ones, where there was lawlessness and the rule of the might.
As for Ukraine and the Russian Federation, such an agreement on the Russian-Ukrainian border, which determined the line of the land border between the countries, was concluded on January 28, 2003 and exists to this day. On the Russian side it was signed by President Vladimir Putin, on the Ukrainian side by President Leonid Kuchma.
https://ru.m.wikipedia.org/wiki/%D0%94%D0%BE%D0%B3%D0%BE%D0%B2%D0%BE%D1%80_%D0%BE_%D1%80 %D0%BE%D1%81%D1%81%D0%B8%D0%B9%D1%81%D0%BA%D0%BE-
%D1%83%D0%BA%D1%80%D0%B0%D0%B8%D0%BD%D1%81%D0%BA%D0%BE%D0%B9_%D0%B3%D1%80%D0 %B0%D0%BD%D0%B8%D1%86%D0%B5
Therefore, an attempt to arbitrarily change the generally recognized borders of Ukraine and Putin’s violation of the previously signed agreement on the land borders of Ukraine is considered aggression and a gross violation of international law with all the ensuing consequences that Russia is already beginning to experience.
That is why Ukraine does not claim the return of territories by Russia, including Moscow itself within the borders of the state of Kievan Rus that once existed on the territory of Ukraine, and even the border Belgorod, Kursk, and Rostov regions that Ukraine had before the 1917 revolution. The above also applies to all states, including the United States with its internationally recognized borders.
In your considerations you miss one important point: the US broke their promise that they would not move NATO to the east anymore.
Ukraine was on the brink of setting up NATO military bases that would point their missiles in the direction of Russia.
The US has already placed a number of bio laboratories in Ukraine that were developing biological weapons against Russia.
CIA has been executing massive information campaigns in Ukraine for at least 10 years that turned Ukraine into anti Russia. As a result masses of Ukrainian population have started hating Russia even before the war started.
Every single day any Ukrainian citizen watching TV was bomparded by anti Russian hate.
What could Putin do with all that threat?
What would the US do if Russia pumped Mexico with tanks and missiles directed at the US, installed bio laboratories that could be developing biological weapons against American citizens, and conduct information campaign whereby every single citizen of Mexico watching TV would see anti American propaganda?