But unyielding as Wittekind was, his great antagonist was equally resolute and persistent. When he had finished his work with the Arabs, he returned to Saxony with his whole army, fought a battle in 779 in the dry bed of the Eder, and in 780 defeated Wittekind and his followers in two great battles, completely disorganizing and discouraging the Saxon bands, and again bringing the whole country under his control. This accomplished, he stationed himself in their country, built numerous fortresses upon the Elbe, and spent the summer of 780 in missionary work, gaining a multitude of converts among the seemingly subdued barbarians. The better to make them content with his rule he treated them with great kindness and affability, and sent among them missionaries of their own race, being the hostages whom he had taken in previous years, and who had been educated in monasteries. All went well, the Saxons were to all appearance in a state of peaceful satisfaction, and Charles felicitated himself that he had finally added Saxony to his empire. He deceived himself sadly. He did not know the spirit of the free-born Saxons, or the unyielding perseverance of their patriotic leader. In the silent depths of their forests, and in the name of their ancient gods, they vowed destruction to the invading Franks, and branded as traitors all those who professed Christianity except as a stratagem to deceive their powerful enemy. Entertaining no suspicion of the true state of affairs, Charlemagne at length left the country, which he fancied to be fully pacified and its people content. With complete confidence in his new subjects, he commissioned his generals, Geil and Adalgis, to march upon the Slavonians beyond the Elbe, who were threatening France with a new barbarian invasion. They soon learned that there was other work to do. In a brief time the irrepressible Wittekind was in the field again, with a new levy of Saxons at his back, and the tranquillity of the land, established at such pains, was once more in peril. Theoderic, one of Charlemagne's principal generals, hastily marched towards them with what men he could raise, and on his way met the army sent to repel the Slavonians. They approached the Saxon host where it lay encamped on the Weser, behind the Sundel mountain, and laid plans to attack it on both sides at once. But jealousy ruined these plans, as it has many other well-laid schemes. The leaders of the Slavonian contingent, eager to rob Theoderic of glory, marched in haste on the Saxons, attacked them in their camp, and were so completely defeated and overthrown that but a moity of their army escaped from the field. The appearance of these fugitives in the camp of Theoderic was the first he knew of the treachery of his fellow generals and their signal punishment.