Carlos I inherited the Spanish kingdoms at age 16 in 1516 and sought to become Holy Roman Emperor. He faced rebellions in Castille and Aragon due to his foreign advisors and tax increases to fund his imperial ambitions. As Emperor, Carlos I sought to unite Europe in a Catholic empire, but faced opposition from Martin Luther's Protestant Reformation, the Pope, the Ottoman Turks, and France. While he had some military successes, the Peace of Augsburg in 1555 marked the failure of his imperial dream. Exhausted from constant warfare, Carlos I abdicated and divided his holdings between his brother and son before retiring.