U.S. security force assistance is trapped in a “Schrödinger’s Cat” paradox: the very metrics used to measure partner military success distort reality and create the illusion of effectiveness. By relying on easily quantifiable indicators—troop numbers trained, equipment delivered, units certified—the U.S. incentivizes performative behavior by both advisors and partner forces, producing polished reports rather than durable institutions. Drawing on examples from Afghanistan, Iraq, the Sahel, and even Ukraine, the authors show how tactical proficiency metrics routinely mask corruption, weak political legitimacy, and institutional fragility, leading to strategic failure despite apparent progress. They contend this problem has worsened under post-2017 assessment frameworks that treat security assistance as a linear, engineering problem rather than a complex adaptive system. The solution, they argue, is not abandoning assessment but redesigning it: shifting from proof-seeking to hypothesis-testing, elevating qualitative advisor judgment, measuring outcomes that partners cannot fake, and aligning evaluation with strategic competition rather than counterterrorism-era outputs—so that when a crisis finally “opens the box,” policymakers aren’t shocked to find a force that only ever looked alive on paper.