Former national security adviser John Bolton was advised to purge potentially biased holdovers from the Obama administration from the Trump White House, according to a top Republican in Congress. House Intelligence Committee ranking member Devin Nunes said on Thursday that he warned Bolton, whom Democrats want to testify in the Senate impeachment trial against President Trump, of an anti-Trump spying operation by members of the National Security Council. The California congressman also said he told H.R. McMaster, who preceded Bolton, and Robert O'Brien, the current national security adviser, to take steps to cull what he called a "den of thieves."...