Darius of Persia maintained control over his empire by the use of provincial governors known as satraps. These satraps ruled areas that were larger than most kingdoms throughout the world, giving them tremendous power and wealth over their division of the empire. Satraps had a number of responsibilities, most notably needing to pay an annual tribute to the emperor and raising soldiers from their province during a time of war. By delegating authority to satraps, Darius was able to make decisions that affected all his empire rather than one region, and whenever he needed a military force (such as during the Persian wars with Greece), he could mobilize the largest army on Earth.