Peacetime military engagement has been a key component of U.S. defense strategy in the post-Cold War era to shape the international environment in ways favorable to U.S. interests. Since September 11, 2001 (9/11), a concerted Department of Defense effort has transformed engagement activities to a broader concept of security cooperation aimed at creating partnerships and building the capacity of allies and partners to meet the challenges of the uncertain and complex security environment. When it comes to security cooperation, however, there will always be a tension between balancing military readiness with security cooperation. Most argue that readiness is the most important priority. But, if adequately funded and properly executed, security cooperation activities may build partners and prevent conflicts. Investing early in shaping activities may avoid exponentially larger expenditures later. In the strategic environment over the next decade, this tension will continue to exist and manifest itself in challenges to security cooperation in resourcing, assessment, and coordination. This paper examines the role of security cooperation in the emerging security environment and the challenges the United States must overcome to be effective.