

Article 5
“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.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.”
https://www.nato.int/cps/ie/natohq/topics_110496.htm
Guess the US had to provide help against themselves in that case. Probably either the alliance as a whole would dissolve or the US would have to leave it.
Yes and per article 5 the US would have to act as if they attacked themselves. Since this is quite silly, an attack of the US against Greenland would most probably either lead to the US having to leave NATO (I don’t know, if there is a way to expel a country from NATO) or NATO dissolving completely.