Ranger students train to exhaustion

The purpose of the U.S. Army's Ranger Course is to prepare these Army volunteers — both Officers and enlisted Soldiers — in combat arms related functional skills. The Rangers' primary mission is to engage in close combat and direct-fire battles.
There are three distinct phases of Ranger School that require Soldiers to make quick decisions in adverse situations these phases are called 'crawl,' 'walk' and 'run.'

 http://www.goarmy.com/soldier-life/being-a-soldier/ongoing-training/specialized-schools/ranger-school.html