History does not run in straight lines. Instead of inevitable progress, what we get is more often false starts, blind alleys, random events, good intentions that go wrong. Robert Cooper's incisive and elegant book is therefore not a continuous diplomatic history. Richelieu and Mazarin inhabited a 16th-century world we can hardly imagine today, but it is from their time that we can begin to see the outline of today's Europe. The Ambassadors includes...