John Henry Temple, better known to students of British history as Lord Palmerston, stood at or near the apex of the British government for nearly 50 years. He was prime minister during the U.S. Civil War and was the powerful and nearly independent-of-everyone foreign minister for 16 years.“Palmy” also had a stint as the home secretary — another of the “four great offices” of the British government, charged with maintaining law and order within the United Kingdom. Palmerston’s early career included two decades as the “secretary of war” — a junior minister at the time, where he was charged with...