Then NATO must engage in military attack directly with Russia plain and simple, which means US will have to fight Russia. If for whatever reason NATO mumbles the response or fails to respond then NATO is effectively dead and can disband altogether.
This is the worry some have here in the Baltics, what would happen if Russia decided to roll in and take some small part first , given we are in NATO would NATO countries be willing to risk a nuclear war with Russia to keep up the 5th article that says that "attack one one is an attack on all"