SAN FRANCISCO/NEW YORK (Reuters) - A closely watched section of the U.S. yield curve inverted on Wednesday for the first time in over 12 years, rattling investors already worried that a U.S.-China trade war might trigger a global recession and kill off a decade-long bull market on Wall Street.The yield on the U.S. 10-year Treasury note tipped 1.4 basis points below 2-year Treasury yields , the first time this spread has been negative since 2007, which was the end of a trend of negative yield..