WASHINGTON — Mike Mullen, a retired U.S. Navy admiral and former chairman of the Joint Chiefs of Staff, said Sunday that President Donald Trump’s threat to revoke the security clearances of former CIA director John O. Brennan and other former officials who have been critical of him is a sign that the president is “creating a list of political enemies.” In an appearance on “Fox News Sunday,” Mullen said that while Trump has the authority to pull the security clearances of former national intelligence and other officials, his doing so is “incredibly problematic.” “It immediately brings back the whole concept of the ‘enemies list’ under President [Richard] Nixon,” Mullen said, adding that it was also reminiscent of the anti-communist crusade led by Joseph McCarthy in the 1950s. Brennan has been among the president’s most vocal critics.