Pope Clement VIII organized a Christian alliance against the Ottoman Empire during the Long War (1591–1606).
Holy League of Pope Clement VIII Wikipedia
With the outbreak of the Long War, Clement VIII sent missions to Emperor Rudolf II, Phillip II of Spain, and other princes. At the beginning of 1594, he sent clergyman Aleksandar Komulović of Nona to central and eastern Europe with the purpose to persuade the rulers of Transylvania, Moldavia, Wallachia and Muscovy to join an alliance against the Ottomans. Komulović also tried to enlist the Zaporizhian Cossacks, who were important as frequent raiders of Ottoman territory. Komulović was to appeal to the Serbs about liberation from the Ottomans. Clement VIII subsidized the Habsburgs with 600,000 scudi in 1594–95. Clement VIII appealed to Spain and Venice in vain. He also hoped that the Swedish king Sigismund II would fight the Ottomans in his role as king of Poland. In 1597, Clement VIII sent a force under his nephew to Hungary. He did it again in 1598. Clement VIII chose not to support the Serb Uprising of 1596–97.
Facilitated by the Pope, a treaty of alliance was signed in Prague by Emperor Rudolf II and Sigismund Báthory of Transylvania in 1595. Aron Vodă of Moldavia and Michael the Brave of Wallachia joined the alliance later that year. Clement VIII himself lent the Emperor valuable assistance in men and money.