NATO was formed in 1949 as a military defense among some of the countries of the free (non-communist) world. Today, it guarantees military assistance to any country that is a part of NATO in case of an international attack on the scale of a war. So, it would probably be good for the US to stay involved in NATO, even just as a precaution in case of an attack from another country, and also to help other countries if they are attacked. Hope this helps!