President Trump has long wanted to control the Federal Reserve, and on Monday night he made his power play by firing Fed Governor Lisa Cook. A central bank with even a semblance of independence may be the casualty. Mr. Trump has resisted his itch to fire Fed Chair Jerome Powell on the savvy advice that he could rattle markets and trigger an extended court battle. But Ms. Cook said Tuesday she’ll challenge her removal, so Mr. Trump will still get his legal fight in what could be a landmark...