Momentous international events that demanded difficult national security decisions also marked Johnson's term. The Berlin Crisis ended in May 1949, when the Russians lifted the blockade. Johnson pointed to the Berlin Airlift as a technological triumph important to the future of air cargo transportation and as an example of the fruits of unification. A week after Johnson took office, the United States and 11 other nations signed the North Atlantic Treaty, creating a regional organization that became the heart of a comprehensive collective security system. After initial reservations, Johnson supported the new alliance and the program of military assistance for NATO and other U.S. allies instituted by the Mutual Defense Assistance Act (1949).