President Trump announced Thursday on social media that he was firing his homeland security secretary, Kristi Noem, and wanted Senator Markwayne Mullin of Oklahoma to replace her. Mr. Trump said Ms. Noem, whose tenure had been marred by a string of controversies, would move into a previously nonexistent security role. Ms. Noem is the first cabinet member to be ousted in Mr. Trump’s second term. She faced intense questioning this week from congressional Republicans about a $220 million advertising campaign in which she figured prominently, and Mr. Trump on Thursday contradicted her testimony that he had signed off on the...