In the 16th century, Spain began to emerge as a unified empire under Catholic monarchs. The marriage of Ferdinand II of Aragon and Isabella I of Castile in 1479 brought the kingdoms together while maintaining their separate institutions. Religious unity was imposed through the Spanish Inquisition beginning in 1492 and the forced conversion or expulsion of Jews and Muslims. The monarchy consolidated power by reducing the power of nobles, controlling the military, limiting city autonomy, and improving tax collection and administration. Through dynastic unions and military campaigns, Spain gained control of territories across Europe and in the Americas, establishing itself as a major world power.