Retired Lt. Gen. H.R. McMaster, a CBS News contributor who served as national security adviser in President Trump's first term, tells "Face the Nation with Margaret Brennan" that this week's removal of Mike Waltz as national security adviser will be "significant" for U.

National security adviser Mike Waltz is leaving his White House post, although soon after reports about his departure were published, President Trump announced he plans to nominate him to be ambassador to the United Nations.

Mike Waltz will be nominated as U.S. ambassador to the U.N., while Secretary of State Marco Rubio will take on the added role of interim national security adviser.