“The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

Each country therefor must consider what it wants to do. It does not automatically mean: hot kinetic phase means you too must enter into a hot kinetic phase.

Instead, a country could consider delivering goods, services, money, equipment, hospitalization etc, without even firing one shot or delivering soldiers, or even participating in such an event.

So, if the USA decreases its contribution to NATO, which the USA by right and treaty can do, the following applies:

If you like NATO, you can keep NATO. Good luck with funding it.