which the access to administrative sources was in any case problematic. Besides that, there are a number of additional problems concerning pre-modern historians; they could be very biased in their reports, as inflating the number of enemies has been one of the favourite propagandistic resources of all times. Even when presenting a balanced account, many historians did not possess military experience, thus they lacked the technical judgement to properly assess and critique their sources. On the other hand, they had access to first hand accounts that could be very interesting, although in the subject of numbers were rarely accurate. Historians consider pre-modern narrative sources to be highly unreliable on the subject of numbers, so that it is not possible to make use of them in a pair to administrative sources. Comparatives between modern and pre-modern periods are thus very difficult. Size of overall armies A clear differentiation should be established between Overall armies, i.e., the overall armed forces of a given political entity, and Field Armies, tactical units capable of moving as a single force along a campaign. The growth in size of overall armies has been considered by several scholars as a key issue of the Military Revolution. There are two main theses: it has been either considered a consequence of the economic and demographic growth of the 17th–18th century or the main cause for the growth of the administration and centralization of the Modern State in the same period. However, some opponents of the general thesis have challenged those views, for instance I.A.A. Thompson has noted how the growth in size of the Spanish army in the 16th–17th centuries contributed rather to the economic collapse of Spain and to the weakness of the central government against regional rebellions while Simon Adams has put in question if there was any growth at all in the first half of the 17th century. The growth is however clear in the second half of the 17th century, when the States embrace the task of recruiting and arming themselves their armies, abandoning the system of commission, prevalent until the end of the Thirty Years' War. The organization of a system of Local and Provincial Militias around this period in several countries (and the growing importance of Local Aristocracy, the so-called "refeudalization of the armies" especially in Eastern Europe) contributed to the extension of manpower base of the national armies, although foreign mercenaries still