Linköping, as Sweden broke free of Poland and of Catholicism. Eighty years before, the Swedish Vasa dynasty had established itself by surviving one bloodbath. Now, it would set its greatest scion in line for the throne by inflicting another. Sigismund was Sweden’s legitimate heir; he was also a characteristic product of dynastic intermarriage whose loyalties were splintered between fiefdoms. But most crucially of all, he had been raised Catholic in a realm turning decisively towards Protestantism. Born of a Polish-Italian mother, he had secured the Polish-Lithuanian throne by election in 1589; when succession added suzerainty of Sweden, his realm was...