Castillo is also the co-director of the Albritton Center for Grand Strategy. He came to the Bush School after serving on the staff of the Strategy and Plans Office in the U.S. Department of Defense. Before he worked at the RAND Corporation and the Institute for Defense Analysis. He holds a PhD in political science from the University of Chicago. His publications include: “Building Foreign Militaries and Learning the Right Lessons from Afghanistan,” Newsweek (August 23, 2021); “The Cold Comfort of Mutually Assured Destruction,” War on the Rocks (June 2021); “Loyalty, Hedging, or Exit: How Weaker Alliance Partners Respond to the Rise of New Threats,” Journal of Strategic Studies (2020); “Don’t Leave Grand Strategy to the Generals,” The National Interest, (November 2019); “Passing the Torch: Implementing a Grand Strategy of Offshore Balancing,” in New Voices in Grand Strategy (Washington, DC: Center for New American Security 2019); “Nuclear Strategies to Deter Conventional Attacks,” in New Perspectives on Coercion, (Oxford: Oxford University Press, 2018); Understanding Russian Nuclear Strategy, (Santa Monica, CA: RAND, 2017); and Endurance and War: The National Sources of Military Cohesion (Palo Alto: Stanford University Press, 2014). His research focuses on U.S. national security policy.