There is no objective standard and no court that decides what action will 'pull is into WWIII'. Maybe German tank shipments will be too much, maybe Starlink terminals, maybe a bad Putin meme. What crosses the line is solely decided by the Russian government, and they do not care at all whether coming generations will deem their actions legitimate or not.

In this case it is extremely dangerous to err on the side of caution, or Putin will be able to apply salami tactics and get whatever he wants.