John Bolton, President Donald Trump’s third and longest-serving national security adviser, is speaking out against his former boss in a bombshell new book about his 17 months at the White House and an exclusive interview with ABC News. A veteran of the past three Republican administrations, Bolton is a well-known and active voice in Republican foreign policy circles who has advocated for hard-line, militaristic positions on North Korea, Iran, Venezuela and other countries. At times during his tenure in the Trump White House, that appeared to put him at odds with Trump the deal-maker.