Pál Szepessy (1636–1685), a Calvinist noble, dedicated his life to the goal of turning Habsburg Hungary into an Ottoman vassal state. He spent much time in the Ottoman world, met with Grand Vizier Ahmed Köprülü (r. 1661–76) on at least four occasions, and regularly visited other Ottoman dignitaries. Szepessy outlined the traumatic effects of Habsburg military occupation and Counter-Reformation on Hungarian society. He was driven by the conviction that God had chosen the Ottomans as his avengers to liberate Hungary from the yoke of the Austrian Antichrist. Szepessy won Köprülü's support for armed uprisings and a guerilla war against the Habsburgs. He also influenced the grand vizier's decision to end the Polish-Ottoman war (1672–76) and turn the Ottoman army against the Habsburg Empire. When Köprülü died in November 1676, the blueprints for Habsburg Hungary's defection to the Ottoman Empire were already in place. They were reactivated by Kara Mustafa in 1682–83. Szepessy's initiatives (and similar efforts by other Hungarians) have been ignored by scholarship; their traces remain dispersed in the Austrian and Hungarian archives. My research draws on Szepessy's appeals to Köprülü; his correspondence with fellow Protestant nobles, family, and Transylvanian supporters; reports of Habsburg spies; dispatches of Habsburg residents at the Porte; and deliberations of the Habsburg War Council. Taken together these sources open a window on the life of an important trans-imperial subject whose life throws into question the clash of civilizations narrative that still dominates much of Habsburg and Central European historical studies.