President-elect Donald Trump on Thursday called for the United States to expand its nuclear arsenal, after Russian President Vladimir Putin said his country’s nuclear potential needs fortifying, raising the specter of a new arms race that would reverse decades of efforts to reduce the number and size of the two countries’ nuclear weapons. In a tweet that offered no details, Trump said, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”