After this the danger-point for the Empire shifted gradually eastwards down the Danube. Claudius had converted Thrace from an allied kingdom into a Roman province in A.D. 46. Much difficulty was caused by the Dacians, who lived just across the Danube on the north bank opposite the Roman province of Mœsia and in the modern Roumania. As the Danube was apt to become frozen in winter it ceased to offer a satisfactory frontier, so long as there were powerful enemies on the other side. At first the Romans tried the system of transplanting them, 50,000 under Augustus and 100,000 under Nero, and settling them in the province of Mœsia. But it was a stupid policy, for it meant constant intrigues between the free barbarians and their enslaved kinsfolk. Vespasian accordingly moved two legions down from Dalmatia to reinforce the two already stationed in Mœsia. But presently there arose an able and heroic king called Decebalus, who welded the Dacians into a compact and organised kingdom, and began to menace the security of the Empire. Like Marbod of Bohemia, he drilled his barbarians on the Roman model. In A.D. 85 he invaded Mœsia, won victories and did great damage. Domitian, called upon to face this peril, was content with inflicting a single defeat upon them and then accepting Decebalus as a client prince. He gave him Roman engineers and artillerymen, and even sent gifts of money which the barbarians were pleased to regard as tribute. This has been set down as cowardice, but it was certainly unwisdom in Domitian, for Decebalus grew stronger and more dangerous. It was left for Trajan, the greatest soldier of all the early emperors, to face this thorny problem in the two great Dacian Wars of 101 and 105 B.C. The whole war is depicted for us by pictures in stone. The spiral reliefs which cover the column of Trajan tell us, with far more detail than the narrative of Dio, the history of the two Dacian Wars. We see the embarkation of the Roman army, we see it on the march with its scouts in advance, we see the solemn purifications, sacrifices, and harangues which preceded battle. We see the battles themselves, in which the Romans with sword and _pilum_ defeat the Dacians and their mail-clad Sarmatian cavalry. The great bridge built across the Danube at Viminacium by the Greek architect Apollodorus is faithfully depicted. We can watch the siege of the Dacian capital, Sarmizegethusa, and observe the construction of the siege-engines. Scenes of pathos are most graphically portrayed, the torturing of Roman prisoners by the barbarian women, the suicide of the Dacian chiefs by poison, and the death of the heroic Decebalus. At intervals throughout the story there appears and reappears the calm and stately figure of Trajan, steering his ship, sacrificing for victory, leading the march or the charge, haranguing his troops, directing the labour of engineering, consulting with his officers, or receiving the submission of the foe.[65]