Octavian was left holding a bag of over 60 legions after Actium, his own and the majority of Antony's legions (which had defected to him). The highly professional, standing army that most people think of when talking of the Roman army is the army of the Principate as reorganized by Augustus, not the army of the Republic, either during the later period or before. Republican armies were raised by magistrates and promagistrates holding imperium according to necessity and then usually disbanded afterwards. This holds equally true for armies after Marius did away with the property requirement and after Sulla's establishment of provincial governorships for propraetors and proconsuls as it did during the earlier period. The only important difference is that campaigns were longer, but the majority of commanders disbanded their armies immediately after their terms of office--Cicero absurdly tried to claim that Piso had transgressed by disbanding his troops before returning to Italy during his proconsulship in Macedonia, claiming that he was trying to prevent his soldiers from letting it be known that he had been disgraced in the field (he hadn't been, and he was simply complying with law). Pompey is really the only exception to this--while holding no official magistracy, Pompey kept his troops under arms (and within Italy) for about a decade or so after returning from the east until Caesar's consulship in 59 ratified the Eastern Settlement and passed the decemvirate. This was not usual, and while armies would exist for the duration of a five-year proconsular command, they were supposed to be disbanded immediately afterwards, and usually were.

Still, the vital fact was that armies could be raised by any magistrate or promagistrate who chose to--and indeed, a promagisterial governor was expected to have an army, it was part of the package. Augustus realized perfectly well the danger this presented (both Caesar and Pompey, after all, had abused this--mostly Pompey, who was not above breaking the law repeatedly to ensure that he kept his army), and set up a number of constitutional reforms to prevent the enlistment of armies by anyone without his immediate authority and approval. Augustus severely downsized the army to 20-something legions, and, despite having no official magistracy, commanded a "special province," consisting of a number of provinces (e.g. Gaul, Spain, Egypt, etc). Most imperial provinces were the frontier provinces, in which military force was necessary--with the exception of Africa, where there was one legion, all legions were stationed in frontier provinces, and no more could be raised without Augustus' approval. The restriction of legionary forces and recruitment was supported by the grant of imperium maius, "greater imperium," to Augustus (despite not holding a magistracy at all, much less one with imperium). Imperium maius basically allowed him to outvote any holder of imperium, meaning that he could command any military commander--and within imperial provinces he could appoint military leaders and provincial administrators, enabling him to restrict commands to those that he trusted.