By June 1948, the Soviet Union, in a move to push the Allies out of Berlin, closed all water, rail, and highway links to the western part of the city. Forced to choose between abandoning West Berlin or supplying all goods by air, the western powers began around-the-clock airlift of vital supplies and material into the beleaguered city. The airlift, nicknamed "Operation Vittles", became the largest air cargo operation of all time. The prime workhorse of the Berlin Airlift was the C-54 Skymaster cargo aircraft, and Kelly was the only depot in the country repairing and overhauling replacement Pratt and Whitney R2000 engines used on the aircraft. By December 31, the Supply Division had shipped 1,317 R2000 engines worth $1.7 million for the airlift.