Russia's offensive all across Ukraine, in response to eight years of localised conflict in the Donbass region bordering Russia, is portrayed by the US and its Western allies as suddenly imperilling a previously reliable "international system".
If the thesis that Russia broke the international system is correct, it would mean other countries may engage in some sort of invasion next time, feeling there are no consequences.
Eight years of Western omissions and callousness on Donbass
Russia explained its actions as an attempt to force an end to eight years of conflict in the Donbass region of Ukraine, bordering Russia. They don't see their actions as the start of a war, but the closing stage of a war between large Ukraine and tiny Donbass. In the latest round of escalation in the region, they say, Ukrainian shells prompted mass evacuations onto Russian territory, and explosions reached Russian territory as the conflict inevitably spilled over their border. The Russians profess a desire to defeat and capture Ukrainian commanders and politicians, with an aim to prosecute them for war crimes over the course of eight years.
The Western perspective is, predictably, more simplistic. They say Russian President Vladimir Putin is personally driven by a desire to restore the Soviet Union, based on selective quotations, and is simply invading Ukraine to accomplish this. They omit all kinds of details. There is no word from them on the suffering of the Donbass region, or they dismiss the entire eight-year conflict there and huge death toll among ethnic Russian residents as Russia's own fault for sympathising with rebels at its border.
Russia's view of the intervention as legal
From the Russian perspective, they are not violating international law but are defending small countries they hastily recognised and signed defence pacts with in the Donbass region - the Lugansk People's Republic and Donetsk People's Republic. The Russians cite the UN Charter, which states that countries are entitled to self-defence and collective defence when attacked from outside. This is dubious, because the LPR and DPR are not member states of the UN, so they are not protected under the Charter. The Charter does seem to allow countries the right to collective defence of allies, though, so Russia likely is referring to this wording to justify its actions.
Could Russia be acting like America?
What we are witnessing is the Russian Federation's own version of NATO's Yugoslav intervention in 1999 and its ultimate revenge for those NATO actions against its Serbian allies. While I don't know enough to comment on military minutiae, it is similar to Yugoslavia: unilateral independence for breakaway territories, a huge campaign targeting the military of the alleged offending regime with the aim to disarm its forces, and an effort to apprehend and prosecute suspected war criminals.
The biggest violator of international law?
Back to the specific issue at hand: the US accusing Russia of being the ones who destroyed the international system and undermined international law.
US and Western bombings of Yugoslavia, Iraq, Libya, Syria, and other countries were major violations of international law, and the US got away without the level of censure Russia is receiving from US allies. Russia cannot be accused of stepping out line, if there is no line. Others were already badly out of line, so staying in line would be suicide for the Russians, especially as these violators venture ever closer to the Russian border. Turkey has a worse record than Russia for breaking international law, entering into the Syrian Civil War and also establishing military positions in Iraq. Its violations are basically identical to Russia's under international law, as are America's.
Russia's rejection of US censure over its intervention by pointing to US violations of international law may be dismissed as "whataboutism", but it is not. Whataboutism is invalid if is merely a sort of insult, and not part of a logical syllogism. In this case, there is a logical syllogism that justifies Russia's temporary dismissal of international law:
Premise 1: The international system depends on people following it (this fits with the US's own statements about Russia threatening the system by not following it), Premise 2: the US and its allies didn't follow international law, Conclusion: the US destroyed the international system
Russia's use of force is the direct result of the US dismissing international law during its own campaigns of conquest and encroachment, forcing others to dismiss it too if their interests are at risk.
Countries are being compelled to use force because since the end of the 1990s, the sole superpower, the US, imposed the law of the jungle (or the state of nature, according to Hobbes) on them with its own capriciousness. "Might makes right" is the idea they propagated, not democracy or the rule of law. If American forces could blow things up and kill people, they were allowed to.
International law has been destroyed by repeat violators, before. When the Axis powers refused to follow international law prior to the Second World War, the Allies were forced to disregard it as well, and invaded a number of countries without sanction under international law as they sought to defend themselves. Russian actions, if examined in isolation, violate international law, but they are a response to continuous violations by the US, who were increasingly interested in training and arming Ukraine on Russia's border.
Note that the sudden appeals to international law by Western countries mark a departure from previous rhetoric on the "rules-based order" - a term specifically devised to avoid following international law. The US has suddenly found itself on the right side of international law in this one case, but their original contributions to this crisis and many other crises consisted of their own violations of international law. The US and its allies have an arsenal of excuses that allow them to invade anyone or do anything they want (human rights, democracy, humanitarianism, doing the right thing etc.). As such, the Americans and their allies weakened the international system and they are at fault for it breaking down and the primacy of force returning as the way to solve major international disputes, which is likely to be seen a lot more in future.
Ukraine crisis was started by US illegal actions in 2014
What is an even better point is that the "democratic" Ukrainian government post-2014 is itself the result of a US violation of international law. In 2014, the United States interfered and directly assisted a coup in Ukraine's capital, Kiev, to steer the country away from Russia regardless of how the population felt. A large following of demonstrators effectively occupied and seized power in the capital, strong-arming the entire nation into following their narrow vision, which aligned just exactly with the US State Department.
Russia could argue that it is unilaterally enforcing international law by removing an illegitimate US-installed government, although this is a bad way to conduct oneself. The reality is that, sometimes, new regimes come to power by force (e.g. Iran's government in 1979 or Yemen's Houthi-dominated government in 2014) and nothing can or should be done.
As far as Russia is concerned, the US has already destroyed the international system as a result of its own interventions in other countries, which violated international law. Russia is still prepared to follow international law in most cases, frequently appealing to it as a series of absolutes from which one can't deviate and aligning with China on this. However, it now seems Russia is prepared to make exceptions to international law when US violations result in escalating violence on an area bordering with Russia.