READ FULL REPORT The United States faces an increasingly contested strategic environment that raises significant, unanswered questions about the role nuclear weapons will play in U.S. national security strategy going forward, and the forces that will be required to deter—and, if necessary, defeat—adversaries. For the first time in its history, the United States must contend with not one but two major nuclear powers. This poses several challenges, chief among them how the United States and its allies might confront simultaneous or sequential high-end conflicts with multiple nuclear-armed adversaries. As policymakers grapple with the “two-near-peer” challenge, a crucial question remains: How should the United States mitigate the risk of opportunistic aggression? To explore these issues and generate new thinking on a central issue affecting future nuclear strategy and posture, the Project on Nuclear Issues (PONI) at the Center for Strategic and International Studies (CSIS) launched the Mid-Career Cadre Task Force focused on Understanding Opportunistic Aggression in the Twenty-First Century. The group, comprising mid-career experts from both governmental and nongovernmental organizations, explored how the United States and its allies can best deter opportunistic aggression; what, if any, changes to force structure and posture are needed to address the two-near-peer problem; and how planning for opportunistic aggression scenarios in the future might affect extended deterrence commitments, strategic stability, and arms control efforts. This report was made possible through general support to CSIS. No direct sponsorship contributed to this report.