Over the centuries, battles have often been won by innovation: cavalry prevailed over infantry, elephants frightened horses, and mechanized vehicles dispersed chariots. Today, aircraft and rockets seem to make victory possible from a safer distance, but we are learning that even overwhelming airpower is not enough. Sooner or later, a war must be fought on the ground. Ground forces often face harsh environmental conditions hot or cold, wet or dry and often the terrain is rugged. The high mountain environment is cold and the air deficient in oxygen. Space travel and underwater operations also pose new and difficult problems. Alexander the Great lost hundreds of men to mountain hazards. Napoleon s attack on Moscow was broken by winter. Mountain combat during both World War I and World War II caused many avoidable casualties from cold and altitude. In the 1962 Sino Indian conflict, these mountain conditions cost the unacclimatized Indian troops more loss than fighting did. And the winter retreat of the US forces during the Korean War was nearly identical except for altitude to that which the British Army had experienced in Afghanistan a century earlier. As this second of three volumes devoted to Medical Aspects of Harsh Environments goes to press, US and Allied forces are preparing to mark the first anniversary of their deployment to the windswept terrain of the Afghan plateau. The reality of American troop operations at 5,000 to 10,000 feet above sea level brings an unexpected timeliness to this volume. For the first time in history, the US military has had to wage sustained combat at high altitude. Troops have had to confront, and overcome, the hobbling effects of altitude sickness while encumbered with 80 lb of gear and negotiating their way across steep snow-covered slopes. One firefight in March 2002 between US Army Rangers and Afghan Taliban took place at 10,200 ft.