President Trump and his aides wasted little time opening a campaign of retribution against those he blames for his impeachment, firing on Friday two of the most prominent witnesses in the inquiry against him barely 48 hours after the Senate acquitted the president. Emboldened by his victory and determined to strike back, Mr. Trump fired Gordon D. Sondland, the ambassador to the European Union, within hours of the White House dismissing Lt. Col. Alexander S. Vindman, a decorated Iraq war veteran who was a Ukraine expert on the National Security Council. Both officials testified to a House committee about the...