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.

President Donald Trump has gone through his share of national security advisers. But he doesn't hold the record, yet.