President Donald Trump said on Thursday he wants to build up the U.S. nuclear arsenal to ensure it is at the "top of the pack," saying the United States has fallen behind in its atomic weapons capacity. In a Reuters interview, Trump also complained about Russian deployment of a cruise missile in violation of an arms control treaty and said he would raise the issue with Russian President Vladimir Putin when and if they meet. On another front, Trump said China could solve the national security challenge posed by North Korea "very easily if they want to," ratcheting up pressure...