Most students of history assume that the age of the "warlord popes" ended with the Renaissance, but, long after the victory of Catholic powers at the Battle of Lepanto in 1571, the Papacy continued to entangle itself in martial affairs. The Vatican participated in six major military campaigns between 1796 and 1870, flew the papal flag over a warship as late as 1878, and during the Second World War mobilized more than 2,000 of its own troops to defend the Pope.

David Alvarez now opens up this little-known aspect of the Papacy in the first general history of the papal armed forces. His is...