So according to this principle, Russia can send military aid to the Syrian government to strike the US military bases on its territory and the US should not be able to retaliate?
Or in an ideal world, the US pays Syria more to not attack them, maybe even gets them to sign a treaty and commits to building Syria's economy and protecting them so they don't feel compelled to take Russian bribes. Although, it's certainly not so straightforward, prior US involvements in foreign countries have been disasters so it would have to be different somehow...
There are many other ways Russia could attack NATO that would be very hard to prove or evenly-counter. Russia could create a culture of NATO hatred and aggression, then set up "rewards" that are given out for obscure reasons, to get Russian NGOs and citizens to attack NATO in their own will. Then NATO can only encourage citizens to attack Russia, which I don't think any treaties forbid anyways, and creating a culture of hate is bad for other reasons. It's not a foolproof system.
But like for this event, there's evidence beyond reasonable doubt that the Russian government is directly involved (https://www.newsweek.com/russia-pipeline-gas-patrushev-putin...). And there are other ways NATO can weaken Russia's influence without even attacking them, like not trading with them, and (indirectly, by having a more liberal government) encouraging Russian citizens to emigrate.
Even worse, inside US and Russian governments there are groups with their own interests and agendas. The military-industrial complex can be interested in further escalation and fearmongering (i.e. "good war"), while civilian industry would prefer some kind of compromise as soon as possible (i.e. "poor peace").