The army of the early empire was sharply and qualitatively different. Augustus defined a permanent professional army by setting the enlistment at 16 years (with an additional 4 for reserve obligations), and establishing a special military fund, the aerarium militare, imposing a 5% inheritance tax and 1% auction sales tax to pay for it.
The Roman army under Augustus was characterized by its professionalization, defined by a permanent force of soldiers with a set enlistment age (16 years) and reserve obligations. This new structure required funding, which Augustus achieved through the aerarium militare, a dedicated fund for military expenses, and taxes on inheritance and auction sales.