I understand the sentiment, but "illegal invasion" has always struck me as a sort of oxymoron... As of there were laws regulating such things.
There are, actually. Created by international treaties and governed by international courts set up specifically to deal with the issues thus arising. (is the International Court of Justice)
For instance, the Hague convention of 1907 says that you shouldn't invade a country without warning; that a declaration of war should be made prior to the use of armed force, and preferably with an ultimatum prior to the use of force stating the reasons for considering the use of armed force to provide a reasonable chance of dealing with issues peacefully via diplomacy <a href="https://pca-cpa.org/en/home/#:~:text=The%20Permanent%20Court%20of%20Arbitration,services%20to%20the%20international%20community>or arbitration</a> rather than resolving issues through bloodshed.
Of course, it only applies to the countries that signed up to it. Russia in fact <b>is</b> one of those countries.