Abstract In 2003, three British reserve army geologists contributed hydrogeological advice to assist planning for the Coalition invasion of Iraq by predicting likely groundwater and drilling conditions. In consequence, 521 Specialist Team Royal Engineers (Water Development) was deployed in theatre soon after hostilities began, to provide a water supply infrastructure for British troops. However, a speedy end to combat, and concentration of British troops in southern Iraq where surface waters were the primary source of supply, necessitated only four new boreholes. Elements of 521 STRE deployed to Afghanistan in 2006, again with hydrogeological guidance, to enhance water supplies for a Provincial Reconstruction Team and Forward Operating Base (FOB), and to develop a water supply infrastructure for the main British operational base at Camp Bastion. Local contractors were used to drill 11 wells, each to over 100 m depth, in Quaternary alluvium. Subsequently, hydrogeology was used to guide successful groundwater development at four out of five FOBs, involving 28 new boreholes, minimizing risks associated with water supply by road or helicopter, and to facilitate expansion of Camp Bastion to accommodate a surge of Coalition troops. Tasks in Afghanistan have generated the most significant British military use of hydrogeology in recent years.