of the Goths, 138. in which the Eastern Emperor Valens was massacred along with tens of thousands of Roman troops. At the same time, Franks raided through the North Sea and the English Channel, Vandals pressed across the Rhine, Iuthungi against the Danube, Iazyges, Carpi and Taifali harassed Dacia, and Gepids joined the Goths and Heruli in attacks round the Black Sea. At around the same time, lesser-known tribes such as the Bavares, Baquates and Quinquegentanei raided Africa. At the start of the 5th century, the pressure on Rome's western borders was growing intense. However, it was not only the western borders that were under threat: Rome was also under threat both internally and on its eastern borders. Usurpers (193–394 AD) An army that was often willing to support its general over its emperor, meant that if commanders could establish sole control of their army, they could usurp the imperial throne from that position. The so-called Crisis of the Third Century describes the turmoil of murder, usurpation and in-fighting that followed the murder of the Emperor Alexander Severus in 235 AD. However, Cassius Dio marks the wider imperial decline as beginning in 180 AD with the ascension of Commodus to the throne, a judgement with which Gibbon concurred, and Matyszak states that "the rot ... had become established long before" even that. Although the crisis of the 3rd century was not the absolute beginning of Rome's decline, it nevertheless did impose a severe strain on the empire as Romans waged war on one another as they had not done since the last days of the Republic. Within the space of a single century, twenty-seven military officers declared themselves emperors and reigned over parts of the empire for months or days, all but two meeting with a violent end.Matyszak, The Enemies of Rome, p. 227 The time was characterized by a Roman army that was as likely to be attacking itself as it was an outside invader, reaching a low point around 258 AD. Ironically, while it was these usurpations that led to the breakup of the Empire during the crisis, it was the strength of several frontier generals that helped reunify the empire through force of arms. The situation was complex, often with three or more usurpers in existence at once. Septimius Severus and Pescennius Niger, both rebel generals declared to be emperors by the troops they commanded, clashed for