Washington — GOP Sen. Bill Cassidy of Louisiana on Wednesday dropped his support for a Democrat-led effort to restrict President Trump’s war powers in Iran after receiving a briefing at the White House — a briefing he said he secured with a note slipped to the president’s envoy to the Middle East following a heated exchange with Mr. Trump behind closed doors. “In one sense, I actually accomplished the mission of what I needed to do,” Cassidy said in an interview Thursday for “Face the Nation with Margaret Brennan.” Cassidy and the president sparred at a testy Senate Republican lunch meeting on Wednesday, where Mr. Trump expressed his discontent with four GOP senators, including Cassidy, who had supported a resolution aimed at reining in the president in Iran a day prior. The