A measure undertaken by the US and British governments to counter the Soviet blockade of Berlin. In June 1948 the USA, Britain, and France announced a currency reform in their zones of occupied Germany. The Soviet Union, fearing this was a prelude to the unification of these zones, retaliated by closing all land and water communication routes from the western zones to Berlin. The western Allies in turn responded by supplying their sectors of Berlin with all necessities by cargo aircraft. The siege lasted until May 1949, when the Russians reopened the surface routes. The blockade confirmed the division of Berlin, and ultimately of Germany, into two administrative units.