As an island within the Soviet Zone of Occupation, West Berlin was connected with the British and American zones through precisely defined routes. This map shows the city's lifelines as of 1948. Traffic proceeded along the roads, railroads, and waterways indicated on the map. These were not established by treaty, however. The movement of traffic was controlled at entrance and exit points to Berlin and the Western zones. The vulnerability of this system was revealed when the Soviet Union blockaded all land routes to the city in June of 1948. For weeks and months, the city could only be supplied through an "air bridge" [Luftbrücke], which followed the three air corridors indicated on the map, routes that were guaranteed by treaty.