NonCredibleDefense

I'm sorry to be the one to break the news to you but every NATO agreement is with the USA as an intermediary. Applications are sent to the USA and approved by the USA.

Without the USA all previous NATO agreements will become void. Until a new organization forms without them, there will be no pact holding the countries responsible to defend each other, and it will likely be difficult to get countries that weren't at immediate risk before to join the new organization given it lacks the USA's firepower as a guarantor against the potential enemies they could make.

Decades of progress put into making the alliance will be gone, and it will become everyone's problem sooner or later. The USA will become a frontline when allies start to fall.