In what treaty did the French, American, and British sectors of Berlin unite and from West Berlin?

Answer 1

"The Federal Republic of Germany, founded on 24 May 1949, was declared "fully sovereign" on 5 May 1955. The former occupying Western troops remained on the ground, now as part of the North Atlantic Treaty Organisation (NATO) which West Germany joined on 9 May 1955, promising to re-arm itself soon.

West Germany became a focus of the Cold War with its juxtaposition to East Germany, a member of the subsequently founded Warsaw Pact. The former capital, Berlin, had also been divided into four sectors, the Western Allies joining their sectors to form West Berlin, while the Soviets held East Berlin. West Berlin was completely surrounded by East German territory and had suffered a Soviet blockade in 1948 which had been overcome by the Berlin airlift."
Wikipedia