His rule over Wallachia began in the autumn of 1593. Two years later, war with the Ottomans began, a conflict in which the Prince fought the Battle of Călugăreni, resulting in a victory against an army nearly three times the size of Michael the Brave, considered one of the most important battles of his reign. Although the Wallachians emerged victorious from the battle, Michael was forced to retreat with his troops and wait for aid from his allies, Prince Sigismund Báthory of Transylvania and Holy Roman Emperor Rudolf II. The war continued until a peace finally emerged in January 1597, but this lasted for only a year and a half. Peace was again reached in late 1599, when Michael was unable to continue the war due to lack of support from his allies. In 1599, Michael won the Battle of Șelimbăr against Sigismund Báthory and soon entered Alba Iulia, becoming the imperial governor (i.e. "de facto" ruler) of Transylvania, under Habsburg suzerainty. A few months later, Michael's troops invaded Moldavia and reached its capital, Iași. The Moldavian leader Ieremia Movilă fled to Poland and Michael was declared Prince of Moldavia. During this period, Michael the Brave changed his seal to represent his personal union of Wallachia, Moldavia, and Transylvania.