Well, just to keep on that narrow focus (and not even invoking anything else), the USA and NATO has violated it multiple times in the last 25 years alone. Let's just take one relatively tame example:

In 1999, NATO bombed Yugoslavia after being denied approval by the UN. Their rationale: "to stop a genocide of Kosovar Albanians" sounds very much like Russia's rationale: "to stop a genocide of ethnic Russians in Donbass". They bombed it for many days, killing hundreds of civilians in the process. Well, thankfully, Russia did not send endless weapons to Yugoslavia to defend itself against NATO, so Yugoslavia surrendered and had to agree to let NATO peacekeepers be stationed in Kosovo. This is not normally considered "NATO occupying part of Yugoslavia", but peacekeepers.

Afterwards Yugoslavia fell apart. Kosovo declared independence unilaterally. This all has parallels with the Ukraine war. https://en.wikipedia.org/wiki/Kosovo_independence_precedent

At the very least, we cannot seriously say that "NATO is a purely defensive alliance". And we can't say "Russia's invasion was unprovoked and unjustified" while simultaneously defending NATO interventions in Kosovo, Libya, Syria, or USA invasions of Iraq, Afghanistan, etc.

Many countries did in fact denounce what NATO was doing. And many countries denounced the invasion of Iraq. However, I have heard people bring up ad-hoc justifications for it like "well, it's not as bad because we didn't want to annex any part of Iraq".