At the end of the day, we should understand that it was the US who provided the quasi-legal justification for Russia's present military incursion into Ukraine.
Under Article 51 of the UN Charter the only legitimate ground for invasion of another country is self-defense against an armed attack--when the danger of
attack is immediate and imminent. Here is what Article 51 reads:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.
That all changed when the US crafted the doctrine of pre-emption--in contravention of Article 51. The latter doctrine was invoked to justify the invasion of Iraq after 9-11. As we now know the major threat issue — that of Iraqi WMD, was a manufactured case for war based on fabricated intelligence. (In a real sense, this was a repetition of the Gulf of Tonkin fabrication that 'justified' the American war of aggression against Vietnam).
The point here is that the US doctrine of pre-emption was specifically intended to justify aggressive war against another country where the threat was not immediate but only anticipated to happen at some future time.
The effect of this doctrine, as William Galston professor of public policy for the University of Maryland said was that "rather than continuing to serve as first among equals in the postwar international system, the United States would act as a law unto itself, creating new rules of international engagement without the consent of other nations."
Fast-forward to Russia's “special military operation” against Ukraine. According to Putin the grounds for this operation were to eliminate a neo-Nazi affiliated military who were accused of carrying out acts of genocide against the Russian-speaking population of the Donbass, and to ensure that Ukraine would never become a de facto member of the NATO military alliance.
The precedent for military aggression based on the doctrine of pre-emption had already been set by the US and gave Putin precisely the argument he needed to justify the "special military operation” against Ukraine. The invocation of pre-emption gave the US a quasi-legal ground for attacking Iraq even though the latter country did not pose an imminent threat to the US. Using the very same logic Ukraine did not need to directly attack Russia but only pose a potential future threat to Russian security.
One could make the case here that unlike the fictional grounds the US invoked for invading Iraq, at least Putin put forward fairly clear evidence that the Ukrainian Army was preparing for a massive military incursion into the Donbass (after bombing the shit out of them for 8 years). Secondly, the real possibility of a NATO-aligned Ukraine would indeed pose a direct threat to Russia--most especially after continuous NATO expansion into Eastern Europe that essentially surrounded Russia. Putin did not need to fabricate this evidence: At the 2008 Summit in Bucharest NATO issued a statement that claimed Ukraine and Georgia would become part of NATO.
Of course, both the military incursions in Iraq and Ukraine should be viewed as crimes of aggression. Moreover, the doctrine of pre-emption and the claim that nation states have the 'right' to 'anticipatory collective self-defense' should be seen as direct violations of international law and in contravention of UN Article 51.
The reality, however, is that the legality of the American doctrine of pre-emption provided Putin with the perfect justification for invading Ukraine. So, the US declaration that the invasion of Ukraine is a war-crime and a violation of Article 51 of the UN Charter, is a stunning example of the pot calling the kettle black.