Ernest Bevin, Labour's postwar foreign secretary, once told the Zionist leader, David Ben-Gurion, that the Balfour Declaration of 1917 was the worst mistake in western foreign policy in the first half of the 20th century. From the perspective of British interests, it was certainly a strategic blunder. It committed Britain to support the establishment of a "national home" for the Jewish people in Palestine when the Jews constituted less than 10% of the population. Britain's promise paved the way for the establishment of the state of Israel, but also unleashed one of the most bitter conflicts of modern times.

The story of the Balfour Declaration has been told many times. Geoffrey Lewis has chosen to focus only on the part played by the two principal architects of the declaration: Arthur Balfour and Chaim Weizmann, the Gentile Zionist and the ardent Jewish nationalist. The result is a perceptive, elegantly written and fair-minded book.

At first sight, Balfour seems an unlikely candidate for the role of mover and shaker. He was a languid aristocrat with a philosophical turn of mind. A popular saying went: "If you want nothing done, Balfour is your man." Yet he was moved by a strong conviction that the case for a Jewish homeland in Palestine was wholly exceptional and that it overrode the natural right of the Arabs to self-determination.

Weizmann, a lecturer in chemistry at Manchester University, was a consummate diplomat and an eloquent advocate who converted many in the British establishment to the Zionist cause. The first meeting between Balfour and Weizmann took place in 1906, three years after the Zionist leadership had turned down the offer of a Jewish homeland in Uganda. Their conversation lasted more than an hour and contained within it the germ of the Balfour Declaration.

Balfour could not understand why the persecuted Russian Jews refused the offer of a safe asylum. Weizmann tried to explain why the Zionists could not accept a home anywhere but Jerusalem. "Suppose," he said, "I were to offer you Paris instead of London." "But, Dr Weizmann, we have London," Balfour replied. "That is true," Weizmann said, "but we had Jerusalem when London was a marsh." "Are there many Jews who think like you?" wondered Balfour. "I believe I speak the minds of millions of Jews," replied Weizmann. "It is curious," Balfour remarked, "the Jews I meet are quite different." "Mr Balfour," said Weizmann, "you meet the wrong kind of Jews."

In fact, most of the leaders of British Jewry were opposed to the idea of a Jewish homeland in Palestine. Prominent among them was Edwin Montagu, the secretary of state for India. Montagu rejected the notion that the Jews were a nation and warned that a Jewish home in Palestine would undermine the struggle for equal rights for Jews in the rest of the world. Balfour, however, was persuaded by Weizmann that race, religion and geography were linked in a unique way for Zionist Jews.

Weizmann's refusal even to look at the Uganda scheme greatly impressed Balfour. He concluded that the Jewish form of patriotism was without parallel, that Zionism was a noble project and that Britain ought to support it on idealistic grounds. This perception led directly to the famous declaration that bore Balfour's name, one that changed the course of Middle East history.

 Avi Shlaim is a professor of international relations at the University of Oxford. His books include Lion of Jordan: King Hussein's Life in War and Peace (Penguin)