Resignation
Around 1973, German security organizations received information that one of Brandt's personal assistants, Günter Guillaume, might be a spy. Brandt was asked to continue work as usual, and he agreed, even taking a private vacation with Guillaume. Guillaume was arrested on April 24, 1974. At the same time, some revelations about Brandt's private life appeared in newspapers. Brandt contemplated suicide and even drafted a suicide note. But he lived on, accepted responsibility and resigned on May 7, 1974.
Guillaume had been a spy for East Germany and was led by Markus Wolf, who later said that the resignation of Brandt was never intended, and that the affair was one of the biggest mistakes of the East German secret service.
Brandt was succeeded as Chancellor by the social democrat Helmut Schmidt. For the rest of his life, Brandt remained suspicious that the fellow social democrat and longtime rival Herbert Wehner had been scheming for his downfall, but evidence for this seems scant.
Late life
After his term as Chancellor, he remained head of his party SPD. Brandt was head of the Socialist International from 1976--1992, working to enlarge that organization beyond the borders of Europe. In 1983, 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 also supported Felipe González' newly legal socialist party in Spain after Franco's death. When Lech Walesa's Solidarity movement organized large strikes in Poland in 1980, Brandt visited with the head of government General Jaruzelski rather than with Walesa.
Brandt was a member of the European Parliament from 1979--1983, and Honorary Chairman of the SPD from 1987--1992.