We engaged in such thinking in the years before the attacks of Sept. 11, 2001; many accepted the conceit that lightning victories could be achieved by small numbers of technologically sophisticated American forces capable of launching precision strikes against enemy targets from safe distances.

These defense theories, associated with the belief that new technology had ushered in a whole new era of war, were then applied to the wars in Afghanistan and Iraq; in both, they clouded our understanding of the conflicts and delayed the development of effective strategies.

Today, budget pressures and the desire to avoid new conflicts have resurrected arguments that emerging technologies — or geopolitical shifts — have ushered in a new era of warfare. Some defense theorists dismiss the difficulties we ran into in Afghanistan and Iraq as aberrations. But they were not aberrations. The best way to guard against a new version of wishful thinking is to understand three age-old truths about war and how our experiences in Afghanistan and Iraq validated their importance.