Operation Allied Force, the 1999 NATO air campaign that sought to prevent a wider humanitarian disaster in Kosovo, represents the triumph of air power to some observers and highlights air power_s limitations for others. While representing a successful cooperative allied military action for NATO, it also suggests limits to U.S.-European military cooperation. This report, a dispassionate assessment of Operation Allied Force, provides perspectives from both sides of the Atlantic as well as political and military implications. The campaign highlighted the growing gap between U.S. military capabilities and those of Europe, the potential consequences of joining a limited-objective operation that expands to undesirable proportions and duration, the absence of consensus both within the U.S. military and the Alliance on the best use of air power, the vulnerabilities of a multimember military coalition engaged in an essentially humanitarian operation facing an adversary fighting for its survival, and the limitations inherent in a fight-and-negotiate strategy that left an unrepentant adversary in power. The report concludes that the European allies can expect continued emphasis on the Defense Capabilities Initiative, a U.S. plan adopted by NATO that stresses the need for all NATO forces to be interoperable, deployable, and sustainable. Furthermore, the Europeans must reverse recent trends of defense reductions and invest more in order to realize major improvements in defense capabilities.