Nobody followed the human rights or no-foreign military intervention chapters of international law, ever. Not in the Cold War [1]. Not in the 90s [2][3].
The one chapter that was held sacrosanct by the Great Powers was no annexing. There was Metternichean proxy governing. But no annexing. That is the red line Putin blundered through.
[1] https://en.m.wikipedia.org/wiki/Foreign_interventions_by_the...