In August 1940 he became a Norwegian citizen, receiving his passport from the Norwegian embassy in Stockholm where he lived until the end of the war.
In 1946, Brandt returned to Berlin, working for the Norwegian government. In 1948, he began his political career with the SDP of Germany in Berlin and became a German citizen again. Outspoken against Soviet oppression during the 1956 Hungarian Revolution and against Khrushchev's 1958 initiative to make Berlin into a "free city", Brandt was considered to belong to the right wing of his party, an assessment that would later change. He was supported by the powerful publisher Axel Springer. From 1957 to 1966, he was Mayor of West Berlin, a particularly stressful time for the city during the construction of the Berlin Wall and the intense political pressures of the Cold War. He became chairman of the SDP in 1964, a post he retained until 1987.
Brandt was the SDP candidate for Chancellor in 1961 and lost to Konrad Adenauer's conservative CDU. In 1965 he ran again, and lost to the popular Ludwig Erhard. But Erhard's government was short-lived, and in 1966, a grand coalition between the SDP and CDU was formed; Brandt became foreign minister and vice chancellor. After the elections of 1969, again with Brandt as the lead candidate, the SDP grew stronger, and after three weeks of negotiations formed a coalition government with the small liberal FDP. Brandt was elected Chancellor on October 21, 1969.
In foreign affairs, Brandt created the new policy of Ostpolitik, involving rapprochement with the German Democratic Republic and detente with the Soviet Union, Poland, and other Eastern Bloc countries. For this work, which normalized East-West relations for the first time since World War II, Brandt was awarded the 1970 Nobel Prize for Peace.
But the policy was highly controversial within West Germany, and several members of his coalition switched sides. In May 1972, the opposition CDU hoped to have the majority in the Bundestag and demanded a vote of the parliament to elect a new Chancellor. To everyone's surprise, the vote failed by an extremely narrow margin; much later it was revealed that two members of the CDU had been paid off by East Germany to vote for Brandt.
In 1973, West German security organizations received information that one of Brandt's personal assistants, Günter Guillaume, might be a spy for East Germany. On April 24th, 1974, Guillaume was arrested on those charges. Amid the turmoil, Brandt resigned on May 17, 1974.
After his term as Chancellor, he remained chairman of the SDP. Brandt was also the head of Socialist International from 1976-1992, working to enlarge that organization beyond the borders of Europe. In 1983, when it was widely feared that Portugal would fall to communism; Brandt supported the democratic socialist party of Soares, which won a major victory, thus keeping Portugal democratic. He had also supported Felipe González' newly legal socialist party in Spain after Franco's death.
Brandt was a member of the European Parliament from 1979-1983, and Honorary Chairman of the SDP from 1987-1992. He passed away on October 8th, 1992, at the age of 78.