As Democratic House Speaker Nancy Pelosi announced an impeachment inquiry into President Trump, she looked into the camera and delivered a sharp ultimatum to Joseph Maguire, the acting director of national intelligence.

Mr. Maguire must turn over the full whistleblower complaint that has plunged Washington into the most serious political standoff of Mr. Trump’s presidency, Mrs. Pelosi said, and he has until his scheduled Thursday appearance before the House Intelligence Committee to do so.

“He...