Whatever West Germans called it, there were endless practical difficulties in coping with a hostile next-door neighbour that was obsessed with spying on every aspect of life, not only internally but externally - particularly since all its 15 million citizens were potential agents because they spoke exacdy the same language - and since a further 4 million of them were inside the gates already. What could such close cousins, who found themselves upon such different paths in life, make of each other? How would they behave as their relationship began to change?

The complexities of the situation were made painfully personal in the story of Willy Brandt and Günter Guillaume. Guillaume, who had crossed over from East Berlin thirteen years earlier, joined the Chancellor's office as a junior aide within weeks of Brandt's election in 1969. For the next four years he served Brandt with devotion and efficiency, and rose to become the personal assistant who organised his travels and accompanied him wherever he went. He was also spying on him, it turned out, with equal devotion and efficiency, on behalf of his employers in the East German Ministry of State Security, and his arrest in 1974 precipitated Brandt's resignation.

Exacdy why it did, though, is by no means obvious. In his official letter of resignation Brandt said that he assumed political responsibility for negligence in connection with the affair. The security services had indeed committed a catalogue of errors in failing to vet Guillaume properly before his appointment, and in allowing him to continue even after the first serious suspicions had been raised against him. In private some of Brandt's colleagues told him that he was open to blackmail by the East German government, because Guillaume had presumably supplied them with a list of Brandt's extra-marital activities. But historians tend to agree that the unmasking of Guillaume was more the occasion of the Chancellor's downfall than its cause. It took its place among a complex of factors arising from the political situation inside the Federal Republic, from the internal conflicts of the SPD and its leaders, from Brandt's own character and physical condition - even from the very success that he had achieved.