The difficulties which Charles Martel had to face were much the same as those which for centuries to follow confronted the sovereigns of western Europe. The great problem of the mediæval ruler was to make his power felt throughout his whole territory in spite of the many rich and ambitious officials, bishops, and abbots who eagerly took advantage of all the king's weaknesses and embarrassments to make themselves practically supreme in their respective districts. [Sidenote: Origin of counts and dukes.] The two classes of officers of which we hear most were the counts (Latin, _comites_) and the dukes (Latin, _duces_). A count ordinarily represented the king within the district comprised in an old municipality of the Empire. Over a number of counts the king might place a duke. Both of these titles were borrowed by the Germans from the names of Roman officials. While the king appointed, and might dismiss, these officers when he pleased, there was a growing tendency for them to hold their positions for life. We find Charles fighting the dukes of Aquitaine, Bavaria, and Alemannia, each of whom was endeavoring to make the territory which he was deputed to rule in the king's interest a separate and independent country under his own supremacy. By successive campaigns against these rebellious magnates, Charles succeeded in reuniting all those outlying districts that tended to forget or ignore their connection with the Frankish empire. [Sidenote: Charles and his bishops.] The bishops proved almost, if not quite, as troublesome to the mayor of the palace as the dukes, and later the counts. It is true that Charles kept the choice of the bishops in his own hands and refused to give to the clergy and people of the diocese the privilege of electing their head, as the rules of the Church prescribed. But when a bishop had once got possession of the lands attached to the bishopric and exercised the wide powers and influence which fell to him, he was often tempted, especially if he were a nobleman, to use his privileged position to establish a practically independent principality. The same was true of the heads of powerful monasteries. These dangerous bishops and abbots Charles deposed in wholesale fashion. He substituted his own friends for them with little regard to the rules of the Church--for instance, he bestowed on his nephew the three bishoprics of Paris, Rouen, and Bayeux, besides two monasteries. The new incumbents were, however, no better than the old; they were, indeed, in spite of their clerical robes, only laymen, who continued to fight and hunt in their customary manner. The most famous of Charles' deeds was his decisive defeat of the advancing Mohammedans who were pressing into Gaul from Spain. Before speaking of this a word must be said of the invaders and their religion, for the Saracens, as the followers of Mohammed were commonly called, will come into our story of western Europe now and then, especially during the Crusades.