That's not how Nato works. There's a process to becoming a member, and if any member says no, it's doesn't happen. It's in the charter. It doesn't apply to a nation already at war. It doesn't apply unless a member is attacked. Nato peacekeeping forces have to be approved by all members, or it's not a Nato force. Think Vietnam, article 5 did not apply. So yes, Ukraine can't be a member unless the United States agrees, and article 5 doesn't apply to non-members.  So basically, good luck with that