As demonstrated by their operations against the Taliban and Al Qaeda, U.S. forces have started to undergo transformation-leaving behind traditional ways of conducting expeditionary warfare and moving toward an integrated force that exploits information technology to gain comprehensive awareness and to operate in networks with speed and precise lethality. Yet European allies continue to focus on stability operations, mainly in or near Europe, and these operations do not pose the severe challenges and dangers that impel force transformation. Because traditional European forces cannot integrate and operate effectively with transformed U.S. forces, the gap in their military cooperability-the ability of coalition forces to function as more or less one force-is becoming unbridgeable, threatening basic U.S. interests, European interests, and the cohesion of the Atlantic Alliance. This report presents a joint German-American analysis of the cooperability challenge. The authors provide a framework for tackling the challenge in the most critical common mission: advanced expeditionary warfare. The key steps, in their view, are for leading European allies to accept this mission, to transform their forces to be able to perform it, and to link their force-transformation process with that of the United States.