The rebellious princes now sought the help of the king of France, Louis IV. (called _d'Outre-mer_, or "from beyond sea," because he had been an exile in England). He marched into Alsatia with a French army, while Duke Eberhard and the Archbishop of Mayence added their forces to those of Giselbert and Henry. All the territory west of the Rhine fell into their hands, and the danger seemed so great that many of the smaller German princes began to waver in their fidelity to Otto. He, however, hastened to Alsatia, defeated the French, and laid siege to the fortress of Breisach (half-way between Strasburg and Basel), although Giselbert was then advancing into Westphalia. A small band who remained true to him met the latter and forced him back upon the Rhine; and there, in a battle fought near Andernach, Eberhard was slain and Giselbert drowned in attempting to fly. This was the turning-point in Otto's fortunes. The French retreated, all the supports of the rebellion fell away from it, and in a short time the king's authority was restored throughout the whole of Germany. These events occurred during the year 939. The following year Otto marched to Paris, which, however, was too strongly fortified to be taken. An irregular war between the two kingdoms lasted for some time longer, and was finally terminated by a personal interview between Otto and Louis IV., at which the ancient boundaries were reaffirmed, Lorraine remaining German. [Sidenote: 940.] Henry, pardoned for the second time, was unable to maintain himself as Duke of Lorraine, to which position Otto had appointed him. Enraged at being set aside, he united with the Archbishop of Mayence in a conspiracy against his brother's life. It was arranged that the murder should be committed during the Easter services, in Quedlinburg. The plot was discovered, the accomplices tried and executed, and Henry thrown into prison. During the celebration of the Christmas mass, in the cathedral at Frankfort, the same year, he suddenly appeared before Otto, and, throwing himself upon his knees before him, prayed for pardon. Otto was magnanimous enough to grant it, and afterwards to forget as well as forgive. He bestowed new favors upon Henry, who never again became unfaithful. During this time the Saxon Counts, Gero and Hermann, had held the Wends and other Slavonic tribes at bay, and gradually filled the conquered territory beyond the Elbe with fortified posts, around which German colonists rapidly clustered. Following the example of Charlemagne, the people were forcibly converted to Christianity, and new churches and monasteries were founded. The Bohemians were made tributary, the Hungarians repelled, and in driving back an invasion of the king of Denmark, Harold Blue-tooth, Otto marched to the extremity of the peninsula of Jutland, and there hurled his spear into the sea, as a sign that he had taken possession of the land.