WASHINGTON — President Obama said Monday that he was revamping American nuclear strategy to substantially narrow the conditions under which the United States would use nuclear weapons, even in self defense. But the president said in an interview that he was carving out an exception for “outliers like Iran and North Korea” that have violated or renounced the main treaty to halt nuclear proliferation. Discussing his approach to nuclear security the day before formally releasing his new strategy, Mr. Obama described his policy as part of a broader effort to edge the world toward making nuclear weapons obsolete, and to...