recognition of Roman authority. The defensive battles that Rome had to endure on the Danube since the 230s, however, paled in comparison to the threat the empire faced in the East. There, Sassanid Persia represented a far greater danger to Rome than the isolated attacks of Germanic tribes. The Sassanids had in 224 and 226 overthrown the Parthian Arsacids, and the Persian King Ardashir I, who also wanted to prove his legitimacy through military successes, had already penetrated into Roman territory at the time of Severus Alexander, probably taking the strategically important cities of Nisibis and Carrhae in 235/236. Economic impact Internally, the empire faced hyperinflation caused by years of coinage devaluation. This had started earlier under the Severan emperors who enlarged the army by one quarter, and doubled the base pay of legionaries. As each of the short-lived emperors took power, they needed ways to raise money quickly to pay the military's "accession bonus" and the easiest way to do so was by inflating the coinage severely, a process made possible by debasing the coinage with bronze and copper. This resulted in runaway rises in prices, and by the time Diocletian came to power, the old coinage of the Roman Empire had nearly collapsed. Some taxes were collected in kind and values often were notional, in bullion or bronze coinage. Real values continued to be figured in gold coinage, but the silver coin, the denarius, used for 300 years, was gone (1 pound of gold = 40 gold aurei = 1,000 denarii = 4,000 sestertii). This currency had almost no value by the end of the third century, and trade was carried out without retail coinage. Breakdown of internal trade network One of the most profound and lasting effects of the Crisis of the Third Century was the disruption of Rome's extensive internal trade network. Ever since the Pax Romana, starting with Augustus, the empire's economy had depended in large part on trade between Mediterranean ports and across the extensive road systems to the Empire's interior. Merchants could travel from one end of the empire to the other in relative safety within a few weeks, moving agricultural goods produced in the provinces to the cities, and manufactured goods produced by the great cities of the East to the more rural provinces. Large estates produced cash crops for export and used the resulting revenues to import food and urban manufactured