President Donald Trump said this week that he wants the Defense Department to begin testing nuclear weapons "immediately," but experts say that's wishful thinking.

President Trump declined Friday to say whether the U.S. will resume underground nuclear weapons testing, which ended in 1992, saying only, “You’ll find out very soon.”

As Trump has toughened both his rhetoric and his stance on Russia, Putin has publicly flexed his nuclear muscles with the test of a new Burevestnik cruise missile on Oct. 21 and nuclear launch drills...