After the Vietnam War, the U.S. military was convinced that a renewed focus on warfighting was vital for its revival. The military's leaders knew they might be ordered to do other things such as peacekeeping and counterinsurgency, but concluded that skilled warfighters could naturally handle these other jobs. There was little need for specialized organizations or technology for operations other than war. Large-scale warfighting became the coin of the realm, defining the U.S. military's spending, training and promotion priorities.
However appealing, this idea was always on shaky ground, since it assumed that ineffectiveness in military activities other than large-scale war was acceptable. When the chances of engaging in conventional warfighting were significant, the United States could reasonably tolerate shortcomings in stability operations, including peacekeeping and counterinsurgency. As the likelihood of a large-scale conventional war declined, however, this made less and less sense. Subsequent peacekeeping operations in the Balkans and counterinsurgency campaigns in Iraq and Afghanistan showed that people and organizations optimized for conventional combat could eventually adapt to stabilization activities, but also that it took time. This had strategic costs, most visibly in Iraq, where the “golden moment” when a major effort might have prevented the spread of insurgency slipped away while the U.S. military reorganized and adjusted.
The peculiar thing is that the United States understood this. A 2005 Department of Defense directive (.pdf), for instance, stated that, "It is Department of Defense policy that stability operations are a core U.S. military mission that the Department of Defense shall be prepared to conduct and support. They shall be given priority comparable to combat operations and be explicitly addressed and integrated across all Department of Defense activities including doctrine, organizations, training, education, exercises, materiel, leadership, personnel, facilities and planning."