If the reason that Putin won't attack NATO countries because he believes the USA will get involved, what is it about US involvement that is the deterrent?
Quantity and quality of the conventional forces in question. The reason NATO had a shot against the vastly larger Red Army was air and naval power.
What we're seeing indicates Russia would get eaten alive in the air, which in turn would lead to massive losses among his armor units and severe difficulties with resupply. If the other side has artillery with shells and tanks with gas, and you don't, you lose in modern warfare.
Russia could still win, if they could prevent the US from getting the stuff across the Atlantic necessary to prosecute a war, but all indications are that their navy isn't up to it.