During the 20th century, nations competed in an arms race that drove the development of progressively more advanced military units. Rather than pay for the continued upkeep of outdated and obsolete weapons systems, advanced nations generally decommissioned or sold off weapons to less well equipped allies. In this way, personnel and resources were freed up to be retrained, rearmed and placed back on the front lines without suffocating the economy in a mire of military support costs.