William G.P. Monahan is a Senior Fellow with the Transatlantic Defense and Security Program at the Center for European Policy Analysis (CEPA).

Monahan has nearly three decades of public service in the Legislative and Executive branches, providing national security, foreign policy, and legal guidance. Monahan is a former Counsel and Policy Director on the Senate Armed Services Committee. His portfolio included a focus on great power competition, including foreign malign influence. He oversaw US Northern Command, US Southern Command, and Department of Defense (DoD) security assistance authorities for building the capabilities of foreign security forces. He also focused on legal issues relating to detention, the laws of armed conflict, and the authorization of the use of military force.

From 2016 to 2017, Monahan served as Deputy Assistant Secretary of State for Political-Military Affairs, overseeing foreign military sales and US security assistance grant programs with allies and partners. Monahan initially served on the SASC staff from 2003 to 2016. During this period, he advised on security and operational matters relating to the conflicts in Iraq and Afghanistan, counterterrorism, and US military force posture for the US European Command and NATO. Before joining the SASC staff, Monahan was a Legal Adviser on arms control and nonproliferation issues at the US Department of State and, before that, the US Arms Control and Disarmament Agency.

He received his B.A. from Yale University and a joint degree combining a master's in Public Affairs from the Princeton School of Public and International Affairs and a law degree from Virginia Law School. He has taught a seminar on the international law of arms control, nonproliferation, and disarmament at the George Washington University School of Law.