 Later, under the general and Emperor Heraclius, the Sassanid Empire of Persia was finally defeated.Late in Heraclius' reign, however, a major new threat suddenly arose to the empire's security in the form of the Saracens.Spurred on by their new religion, Islam, which demanded the subjugation of the world or its conversion to "dar al-Islam", driven by a still-strong tribal warfare mentality, and under the brilliant leadership of Khalid ibn al-Walid, these invaders rapidly overran many of the empire's wealthiest and most important regions, especially Syria, the Levant and Egypt.