The UK and France nevertheless became close as both feared the Americans would withdraw from Europe leaving them vulnerable to the Soviet Union's expanding communist bloc. The UK was successful in strongly advocating that France be given a zone of occupied Germany. Both states were amongst the five Permanent Members of the new UN Security Council, where they commonly collaborated. However, France was bitter when the United States and Britain refused to share atomic secrets with it. An American operation to use air strikes (including the potential use of tactical nuclear weapons) during the climax of the Battle of Dien Bien Phu in May 1954 was cancelled because of opposition by the British. The upshot was France developed its own nuclear weapons and delivery systems.