Bolton says willing to testify in Senate trial of Trump's impeachment if subpoenaed

File photo taken on Aug. 2, 2018 shows John Bolton addressing a press briefing at the White House in Washington D.C., the United States. Former U.S. National Security Adviser John Bolton said on Jan. 6, 2020 that he was willing to testify in the Senate trial of President Donald Trump's impeachment if subpoenaed. (Xinhua/Liu Jie)