In this paper I study the question: under what conditions and through which mechanisms does United States military foreign security assistance (mFSA) during active conflicts lead to weapons leakage to insurgent and terrorist groups? To answer this question, I formulate an explanatory theory of weapons leakage, or the unintended proliferation of weapons meant for mFSA, and complete three case studies as a plausibility probe of the proposed theory. These cases include Afghanistan during the Soviet invasion (1979-1989), Afghanistan during the Global War on Terror (2001-2021), Iraq since 2003, and the war in Ukraine (2022...). I conclude that weapons leakage to insurgent and terrorist groups during active conflicts is most likely to occur under the condition of a "fragmented US proxy war," which contains six potential mechanisms that facilitate weapons leakage: lack of intelligence on the actors receiving mFSA, lack of direct control over mFSA weapons provided, lack of reliable weapons-tracking post FSA, reliance on unstable foreign governments, poor oversight and unreliable weapons tracking systems, and corruption resulting from counterinsurgency and counter-terrorism efforts.