I have a huge problem with bringing up the behaviour of countries from centuries ago in this day and age.
One can justify any silly and agressive behaviour with it. Back in it's day the Chinese were not shy either to act in a very imperial way to their neighbours like Korea or Vietnam. In shorter history they annexed territory like Tibet.
Russias recent past is even less stellar with their unorovoked attacks on Finland, the Baltics and Poland during WWII with putting the iron boot onto eastern Europe following immediately after WWII.
We could just as well start to pummel the North African states for their conquest of the Iberian peninsula or the Turks for their takes on conquering Vienna. Or maybe the French for their conquest spree after the revolution or the Scandinaviand for their viking raids.
You see were this leads to...
I don't care if a country doesn't feel great anymore and can't come to grips with it's declined importance. One doesn't need to be the big stomping bear or dragon in order to provide a good live for one's citizens. In fact diverting the resources for the improvement of the living standard is much more benefitial to a country.
Three things.
First, it's not so much about justification. Those come later. It's about reasons for action. It matters far more
why China is acting the way it is then how it justifies those actions.
Second, when actions in relatively recent past (like the post-Cold War settlement) have a giant impact today, pretending that the past is the past isn't really fair. And comparing the treatment of China over the late 19th and first half of the 20th century to something that happened by geopolitical entities that haven't been around for centuries isn't really relevant. The US has a whole network of small islands in the Pacific. That isn't the historic result of some ancient events. It's the direct result of a deliberate imperialist policy in modern history. I'm not saying the US needs to hand over those islands, but I am saying that Chinese desires are modest compared to what the US
already has. Chinese behavior is problematic not only because of the behavior but also because there is no currently acceptable international framework to deal with a rising power without conflict.
Third, it's not so much about countries losing importance. That's a significantly smaller problem. If Russia was getting weaker and feeling insecure about it, it would be no big deal. It's the fact that Russia was weak and is getting stronger again, and wants to undo what was forced (or "persuaded" in exchange for promises made and not always kept) on it when it was weak. Much the same but on a bigger scale applies to China. And unfortunately there is the Middle-Eastern powderkeg, a startling and very unpleasant parallel to the pre-WWI Balkans.
It seems to me Waylander that you are approaching the problem in terms of right and wrong. But these are not useful categories when we are talking about politics. It doesn't matter how "right" the US position is vis-a-vis international law (that the US had a major hand in writing, and China got little to no say in

) if the US being right still leads the world into WWIII with a tens of millions death toll, and entire countries laid to waste. This is why I think situations like Ukraine and the SCS are alarm bells that the current order is in trouble, and the process of looking for a new one needs to begin.