Not long after Michael became Prince of Wallachia, he turned against the Ottoman Empire. The next year he joined the Christian alliance of European powers formed by Pope Clement VIII against the Turks, and signed treaties with his neighbours: Sigismund Báthory of Transylvania, Aaron the Tyrant of Moldavia and the Holy Roman Emperor, Rudolf II (see Holy League of Pope Clement VIII). He started a campaign against the Turks in the autumn of 1594, conquering several citadels near the Danube, including Giurgiu, Brăila, Hârşova, and Silistra, while his Moldavian allies defeated the Turks in Iaşi and other parts of Moldavia. Mihai continued his attacks deep within the Ottoman Empire, taking the forts of Nicopolis, Ribnic, and Chilia and even reaching as far as Adrianople.