This chapter examines the Lend-Lease Act and its effectiveness in supporting the Allies during World War II (WWII), including Yugoslav resistance forces, by facilitating economic and military mobilization to counter Axis aggression. Following the occupation of Albania by Fascist Italy in April 1939, the Axis powers, through the Tripartite Pact signed in September 1940, sought to consolidate their influence in the Balkans. Yugoslavia, under the regency of Prince Paul, eventually signed the Pact in March 1941 under significant pressure, prompting a rapid deterioration of internal political stability. Throughout this chapter, the author examines the kingdom’s ethnic structure and the prominence of Serbian nationalist organizations—such as the pre-war National Defense and the Serbian Revolutionary Organization, which had historically sought the expulsion of the Ottoman Empire from Kosovo—shaped both political and paramilitary developments. The situation escalated when Yugoslav military officers, supported by nationalist elements, carried out a coup d’état on 27 March 1941, repudiating the Tripartite Pact and triggering Axis military intervention. By the time of the Tehran Conference in late 1943, Allied support had shifted decisively toward Josip Broz Tito’s Partisans, who were recognized as the primary Yugoslav resistance force against the Axis, reflecting the Allies’ strategic reassessment of effective resistance movements in the region.