NATO is a treaty ratified by the U.S. Senate in 1949. I don’t think that a President can unilaterally pull out of a treaty obligation.
A POTUS can violate an existing treaty, rendering it null and void.
And nobody can stop him as long as he holds the office.
Even if removed from office, that treaty is still void due to violation.
It would have to be redone.
The agreement falls apart if the member states refuse to adhere to the rules and obligations agreed to.