War by Contract, Credit and Contribution: The Thirty Years War

By any realistic standard waging war on this scale was beyond the capacity of Sweden, a poor northern country with limited natural resources and a population estimated at no more than 1.3 million...That she nevertheless did so illustrates the far-reaching effects of three developments in the organization of early modern warfare, which in combination not only enabled Sweden to achieve disproportionate military strength, but also sustained all the principal participants in the largest and longest war in Europe up to that time. These were the system of military contracting, which allowed princes (and other rulers or states) to engage large numbers of mercenary troops quickly and easily, the availability of sources of credit, which enabled them to put armies in the field having barely paid a deposit on them, and the so-called contributions, which forced the populations of occupied territories to bear the main costs of war [Author, pg. 101]