This dissertation is a quantitative analysis of language proficiencies through the lens of the dynamic model of multilingualism, using survival analysis to evaluate time until thresholds of foreign language proficiency are reached and nonparametric tests (Wilcoxon signed rank and Kruskal-Wallis) to compare proficiencies at distinct time intervals after acquisition. My original contribution to knowledge is demonstrated through the empirical analysis investigating the role of prior language knowledge in the development and sustainment of a multilingual repertoire. I demonstrate that (a) the level, not merely the fact, of prior language proficiency attainment is a significant driver of L3 outcomes even when the L2 and L3 are unrelated and (b) multiple language systems cannot be maintained over a long period of time without significant effort. As a project explicitly addressing multilingualism, this study evaluates changes in languages both before and after a new language has been studied to advanced levels. These findings have important implications both in the context of the community of US military language professionals centered in this study, as well as for teachers and students of multiple languages in that it highlights the central role of proficiency attainment during multilingual study. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]