"Let it be an arms race."That's how President-elect Donald Trump clarified a tweet he sent Thursday which said the U.S. must "strengthen and expand" its nuclear arsenal, a remark that set off warnings and calls for explanation. He then spoke on the phone with MSNBC's "Morning Joe" co-host Mika Brzezinski, who reported on air Friday morning that the president-elect said, "Let it be an arms race. We will outmatch them at every pass."