Edward Gibbon famously levelled a harsh indictment of Septimius Severus as a principal agent in the empire's decline. "The contemporaries of Severus, in the enjoyment of the peace and glory of his reign, forgave the cruelties by which it had been introduced. Posterity, who experienced the fatal effects of his maxims and example, justly considered him as the principal author of the decline of the Roman empire." According to Gibbon, "his daring ambition was never diverted from its steady course by the allurements of pleasure, the apprehension of danger, or the feelings of humanity." His enlargement of the Limes Tripolitanus secured Africa, the agricultural base of the empire where he was born. His victory over the Parthian Empire was for a time decisive, securing Nisibis and Singara for the empire and establishing a "status quo" of Roman dominance in the region until 251. His policy of an expanded and better-rewarded army was criticised by his contemporaries Cassius Dio and Herodianus: in particular, they pointed out the increasing burden, in the form of taxes and services, the civilian population had to bear to maintain the new and better paid army. The large and ongoing increase in military expenditure caused problems for all of his successors.