Article 5 may not come into it. Russia may perceive an attack from Poland (say) as an attack from all of NATO.International law wise, they are right. Providing even just logistical support from your bases makes you a party to the war. So the countries that allow Ukrainian aircraft to use their bases have already joined the war on the Ukrainian side before getting attacked by Russia. This also makes the NATO article 5 non-applicable.
The problem is the US would have a very hard time explaining this to its own populace and others. People think any conflict that has a NATO member in must pull all the others in too. If the US joins the war after such an incident, it will be risking its own existence. If it doesn't (which it shouldn't according to even NATO treaty itself) it will lose a lot of its credibility in an unjustified way.
I can see the US having a very serious conversation with the Baltics, Romania, and Poland right now if the allegations are real. The US is a lot of things but it isn't a country that would allow Eastern European NATO members to skirt around WW3 hiding behind its might.
Those conversations have already happened and appear to have had the intended effect. Poland was previously baying for Russian blood, they've suddenly lost all interest in sending jets to Ukraine.
It is a very difficult situation for Biden that I don't envy. Not only does he need to manage the expectations of his own population who are very anti-Russian right now. But also the governments and electorate of every NATO country. It only takes one to do something stupid and we are in a potential nuclear war.
Most of the population are completely unaware how close we came to nuclear war a week ago, many people in the western world aren't aware that nuclear war is even being discussed, despite that fact Russia has publically threatened it on multiple platforms.
The only problem with this is that there is no underlying ideological underpinning for an alliance. Being anti-western isn't an ideology. Putin only dislikes the west because of years of russophobia. This isn't a country like Iran which is anti-western for religious reasons, or North Korea or Vietnam which are communist.Sino-Ruso relationship is defined by China as 新时代全面战略协作伙伴关系, Full spectrum strategic "collaboration" partnership. It is the highest bilateral relationship in China's diplomacy. I don't know about the translation of 协作 to collaboration is accurate in English or not. But in Chinese definition it is a closer and fuller cooperation that involves one party to assist the other party's strategic objective and vis versa.
It is an alliance without the word "alliance". So in essence Russia and China are strategic allies. I am confident that the Chinese leadership knows what they are doing, so is the Russian leadership, and they are well aware of the histories.
What if tomorrow NATO offer Putin a deal to take over CIS countries and recreate the Russian Empire in return for support against China? Would he take it? Maybe he would, maybe he wouldn't. But he doesn't have an ideological reason to say no.
That's why I think there will be a deal between Xi and Putin for Russia for the communist party to take over Russia when Putin retires.