Notes the increasing gap in the ability of the U. S. and European militaries to operate effectively with each other, and examines ways to address the challenge. As the U. S. military transforms into a highly mobile and lethal force designed for expeditionary warfare, European allies remain focused on stability operations. Because traditional forces cannot integrate or function effectively with transformed forces, the U. S.-allied cooperability gap is becoming unbridgeable, threatening basic U. S. and European interests and alliance cohesion. The authors explore strategies for addressing this challenge.