John III (Swedish: Johan III, Finnish: Juhana III; 20 December 1537 – 17 November 1592) was King of Sweden from 1569 until his death. He was the son of King Gustav I of Sweden and his second wife Margaret Leijonhufvud. He was also, quite autonomously, the ruler of Finland, as Duke John from 1556 to 1563. In 1581 he assumed also the title Grand Prince of Finland. He attained the Swedish throne after a rebellion against his half-brother Eric XIV. He is mainly remembered for his attempts to close the gap between the newly established Lutheran Church of Sweden and the Catholic church. His first wife was Catherine Jagellonica of the Polish-Lithuanian ruling family, and their son Sigismund eventually ascended both the Polish-Lithuanian and Swedish thrones. He was the second son of Gustav Vasa (1523–60). His mother was Margaret Leijonhufvud (1514–51), a Swedish noblewoman. As a Duke of Finland, he opposed his half-brother Eric XIV of Sweden (1560–68) and was imprisoned in 1563. After his release from prison, probably because of his brother's insanity, John again joined the opposition, deposed Eric and made himself the king. His important ally was his maternal uncle Sten Leijonhufvud, who at deathbed was made Count of Raseborg. Shortly after this John executed his brother's most trusted counsellor, Jöran Persson, whom he held largely responsible for his harsh treatment while in prison. John further initiated peace talks with Denmark and Lübeck to end the Scandinavian Seven Years' War, but rejected the resulting Treaties of Roskilde (1568) where his envoys had accepted far-reaching Danish demands. After two more years of fighting, this war was concluded without many Swedish concessions in the Treaty of Stettin (1570). During the following years he successfully fought Russia in the Livonian War, concluded by the Treaty of Plussa in 1583, a war that meant a Swedish reconquest of Narva. As a whole his foreign policy was affected by his connection to Poland of which country his son Sigismund III Vasa was made king in 1587. In domestic politics John showed clear Catholic sympathies, inspired by his Polish wife, a fact that created frictions to the Swedish clergy and nobility. He sought to enlist the help of the papacy in gaining release of his wife's family assets, which were frozen in Naples. He also allowed Jesuits to secretly staff the Royal Theological College in Stockholm. However, John himself was a learned follower of the mediating theologian George Cassander. He sought reconciliation between Rome and Wittenberg on the basis of the consensus of the first five centuries of Christianity. John approved the publication of the Lutheran Swedish Church Order of Archbishop Laurentius Petri in 1571 but also got the church to approve an addendum to the church order in 1575, Nova ordinantia ecclesiastica that displayed a return to patristic sources. This set the stage for his promulgation of the Swedish-Latin Red Book, which reintroduced several Catholic customs and resulted in the Liturgical Struggle, which was not to end for twenty years. In 1575, he gave his permission for the remaining Catholic convents in Sweden to start receiving novices again. From time to time he was also at odds theologically with his younger brother Duke Charles of Sudermannia (afterwards Charles IX of Sweden), who had Calvinist sympathies, and did not promote King John's Liturgy in his duchy. John III was an eager patron of art and architecture.

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