Is America's word worthless?

If the Trump government breaks the Joint Comprehensive Plan of Action (JCPOA), a  25-year agreement limiting Iran's nuclear capacity it entered into in July 2015, how will it be able to enter into similar agreements with other countries, such as North Korea?

Since Trump will have conclusively demonstrated that America's word is worthless the only rational course of action for both Iran and North Korea is to develop a full arsenal of nuclear weapons and be willing to use them.