Historians generally accept that the practice of sustaining armies by exacting forced "contributions" from occupied lands became endemic during the Thirty Years War (1618-1648). Historians have hotly debated, however, the question of how the contribution system affected the progress of the war. This article approaches the problem through a case study of a plan by Cardinal Mazarin to raise troops from the neutral bishopric of Liège in 1643-44. In addition to highlighting the surprising difficulty of recruitment by this stage of the war, this study also demonstrates that contemporaries modified their military strategies to fit their belief that the contribution system could sustain only a limited number of troops (though how many troops that might be was an open question). Yet the contribution system influenced not just the progress but also the length of the war, since rulers dependent on contributions could maintain their war effort at a certain level almost indefinitely, but could never support an army large enough to overwhelm their enemies.