31. So far we have spoken only of the relations of Charlemagne with the Germans, for even the Lombard kingdom was established by the Germans. He had, however, other peoples to deal with, especially the Slavs on the east (who were one day to build up the kingdoms of Poland, Bohemia, and the vast Russian empire) and, on the opposite boundary of his dominion, the Arabs in Spain. Against these it was necessary to protect his realms, and the second part of Charlemagne's reign was devoted to what may be called his foreign policy. A single campaign in 789 seems to have sufficed to subdue the Slavs, who lay to the north and east of the Saxons, and to force the Bohemians to acknowledge the supremacy of the Frankish king and pay tribute to him. [Sidenote: The marches and margraves.] The necessity of insuring the Frankish realms against any new uprising of these non-German nations led to the establishment, on the confines of the kingdom, of _marches_, i.e., districts under the military control of counts of the march, or _margraves_.[42] Their business was to prevent any hostile incursions into the interior of the kingdom. Much depended upon the efficiency of these men; in many cases they founded powerful families and later helped to disintegrate the Empire by establishing themselves as practically independent rulers. [Sidenote: Charlemagne in Spain.] At an assembly that Charlemagne held in 777, ambassadors appeared before him from certain disaffected Mohammedans. They had fallen out with the emir of Cordova[43] and now offered to become the faithful subjects of Charlemagne if he would come to their aid. In consequence, he undertook his first expedition to Spain in the following year. The district north of the Ebro was conquered by the Franks after some years of war, and Charlemagne established the Spanish March.[44] In this way he began that gradual expulsion of the Mohammedans from the peninsula which was to be carried on by slowly extending conquests until 1492, when Granada, the last Mohammedan stronghold, fell.[45] [Sidenote: Charlemagne crowned emperor by the pope.] 32. But the most famous of all the achievements of Charlemagne was his reëstablishment of the Western Empire in the year 800. It came about in this wise. Charlemagne went to Rome in that year to settle a controversy between Pope Leo III and his enemies. To celebrate the satisfactory adjustment of the dispute, the pope held a solemn service on Christmas day in St. Peter's. As Charlemagne was kneeling before the altar during this service, the pope approached him and set a crown upon his head, saluting him, amid the acclamation of those present, as "Emperor of the Romans." [Sidenote: Charlemagne merited the title of emperor.]