 However, once the decision was taken to send a corps-size field force to fight in the South African War, the system began to show a strain.Three divisions were earmarked for deployment in October 1899, and 25,000 men were mobilised from the reserve to reinforce them; however, regulations of the time held that men under twenty should not be sent overseas, so the reservists had to replace underage or otherwise ineffective soldiers as well as filling the existing gaps.In some units, this meant reservists made up over 50% of the battalion strength; throughout the force, 36.5% of men were listed as unfit for foreign service.