War disciplines militaries: it forces them to refine, and sometimes revise, their tactics, techniques and technologies, or risk defeat in battle. Yet there is no theory of how militaries improve in war. This article develops a theory of military adaptation, which it applies to an analysis of the British campaign in Helmand from 2006 to 2009. Drawing on a wealth of primary sources (military plans, post operation reports and interviews), it shows how British brigades adapted different ways of using combat power to try and defeat the Taliban from 2006-07, and how from late 2007, British brigades have adapted a new population-centric approach that has focused more on influence operations and non-kinetic activities.