Given China's recent embargo of exports of rare earth minerals, concern for U.S. vulnerability to supply disruptions for defense-critical minerals is growing. In this study, I test my hypothesis that in practice a combination of economic, geographic, and political factors is usually necessary for minerals supply disruptions to affect the U.S. defense industrial base. In order to answer this question, I compare cases of four minerals - gallium, rhenium, tantalum, niobium - and rare earth elements, focusing from 2005 to the present. Each of these is identified in the 2008 National Academies of Science report, "Managing Materials for a Twenty-First Century Military," as critical to military assets that will grow in importance in future warfare. Among these cases, I will compare the potential causes of supply disruptions in order to determine what factors are most important in signaling vulnerability. This initial study appears to confirm the importance of creating a suite of policy prescriptions that address a range of potential vulnerabilities. The next logical steps in building on this work include examining minerals exported from the Democratic Republic of Congo (such as coltan) and lithium, given growing concerns for the vulnerability of supplies from Bolivia.