One should never overlook the horrors of peace. Between 1750 and 1815, according to the best modern historians, the “Barbary” states—actually the North African states of the Ottoman Empire—took as many as a million and a half Americans and Europeans into slavery. These monarchies—today’s Algeria, Morocco, Libya, and Tunisia—also took part in a north-south version of the Atlantic African slave trade, but their attitude to their “white” or “Christian” captives was slightly different. For a price, these could be redeemed. For a higher tariff, the pirate states would agree to abstain from taking ships or hostages in the first place. This latter price had a tendency to increase the more often it was paid.

There was also a slave-market at Malta for North Africans, operated by Christian merchants, because this conflict had been going on intermittently, in the Mediterranean, ever since the battle of Lepanto. But when Thomas Jefferson and John Adams waited upon a Barbary ambassador in London, in the years after the revolution, they pointed out that the United States had no quarrel with the Muslim world. Oh, yes it did, replied the ambassador. The Qur’an gave permission, as of right, to plunder and enslave all unbelievers.

The Barbary states probably had an imperfect idea of the potential strength of the United States. At all events, they were surprised when, beginning with the Jefferson administration, naval squadrons began to appear off their coasts and to demand, at cannon-point, the liberation of the hostages as well as free trade and free passage from the Atlantic. This was America’s first war overseas and the first time that the Stars and Stripes were planted on foreign soil. I had known that the first line of the Marine Corps hymn, “From the halls of Montezuma to the shores of Tripoli,” derived from this episode. I had not known, until I read London’s book, that Francis Scott Key adapted The Star-Spangled Banner from a song he had written in celebration of William Eaton’s victory over the sultan of Tripoli in 1805.

There is much else to be learned from London’s impressive study, which not only describes the campaign itself in enthralling detail but also analyzes the war within the war: the many domestic disputes and political quarrels that delayed final victory. (Mr. Jefferson does not come off as well as I had previously thought.) Here is an essential piece of U.S. history that—like the name Stephen Decatur—is no longer taught in school. By the end of the conflict, not only did the Muslim world know that it had to reckon with the United States but so did the European powers who—having long paid the “tribute” to Barbary—had just exhausted themselves at Waterloo. Moreover, the new republic had acquired a battle-hardened navy and marine corps, which was to be of great service in the war of 1812 and beyond. Victory in Tripoli was in every sense a hinge event.