They already have some boots on the ground. But official involvement of other nations will definitely raise the level of violence from "special operation" to "local war" (with direct military attacks against Polish and German military force and industrial infrastructure) or even to "regional war" (with usage of tactical nuclear weapons).
Yes off course NATO has units in Ukraine - already way before the war. That especially NATO SpecOps, medical and logistic personal has been increased in Ukraine is also a fact. But there are no NATO battle groups in Ukraine - so far.
International law is defined solely by the USA and it's affiliates. Therefore even if e.g. Russia or China would protest - nobody would give a shit. As such e.g. Poland is free to send troops into Ukraine. As long as they only fight within Ukraine's internationally defined and acquitted territory - Russia could "personally" define this as a deceleration of war towards Russia, or as an act of war - but it wouldn't be according to international law. "They" would make sure of that.
After all it was Russia that attacked a sovereign country - and not the UAF attacking Russia and occupying Russian territory. !!! - that is what Putin always likes to forget.
Vietnam in 1979 was officially persecuting, disowning and killing Vietnamese people of Chinese heritage in Vietnam - resulting in a Boat-People II issue with an estimated 800,000 refugees. Additionally Vietnam attacked an official ally of China - right onto occupying it's capital - and until today the West claims that China attacked Vietnam !!! - and no one gives a shit about the reasons nor the factual occurrences.
Yes Putin could retaliate with e.g. nuclear strikes onto Ukrainian territory - without thereby declaring a war against e.g. Poland or constituting a war towards Poland - due to Polish troops fighting in Ukraine, and even without e.g. Polish troops fighting there - if Putin would launch an attack onto Poland - it would constitute the basis for NATO's article 5. As such IMO Putin will NEVER use nukes against a NATO members territory, nor attack NATO territory even conventionally. And NATO knows this very well.
And that is why NATO will have to enforce a peace-treaty onto Ukraine - otherwise they can't get Ukraine into NATO and the Goebbels scholar dream will never come true - and Putin knows that very well. As such he will cling onto as much Ukrainian territory as possible. How much he can and want's to spend/afford to cling onto territory - only he knows.
So the only question to me now is:
Will Ukraine be able to "liberate" occupied territory before acquitting towards a peace-treaty, based onto their own military, or will some NATO states send in conventional troops to aid in this quest. And IMO, those stupid and spineless NATO *****, should have done that already in Feb. 2022. After-all, they NATO and Ukraine wanting to become a NATO member, caused this war in the first place.