AS a five-year-old playing in New York's Central Park, Henry Morgenthau III was asked by a little girl what his religion was. When he got home, he put the question to his mother. If anyone ever asks you that again, she replied, just tell them you're an American.Now a grown man with children of his own, Henry Morgenthau III does not recollect "ever being told in so many words" that he was Jewish. "It just seeped into my consciousness." The story of the Morgenthau family is on one level a classic American saga of opportunity, success, and assimilation. But, as Henry Morgenthau III makes clear in this absorbing and appealing family history, it is also the story of complex social adjustments and intergenerational tension, of growing confidence mixed with lingering insecurities, of ambition, idealism, optimism, and some keen disappointments. The author begins with his great-grandfather, Lazarus Morgenthau, a penniless German Jew who got his start as a tailor's apprentice. Ambitious, hard-working, and quick to grasp business opportunities, he eventually made his fortune from manufacturing cigars. Business reverses prompted him to emigrate to America in 1866, where he got something of a fresh start, but continued to live beyond his means. His loyal and long-suffering wife was reduced to taking in boarders. Most of his children had to sacrifice their education to take jobs, including his son Henry Morgenthau, who struggled to become a lawyer. Henry, who had been born in Germany in 1856, resented his father's erratic and irresponsible behavior, yet derived a sense of optimism from his childhood memories of prosperity. Henry was a classic Victorian man: energetic, hard-working, solid, and cheerful. He was determined to avoid his father's mistakes and to live up to his mother's fine moral principles. Although Lazarus had achieved surprising success - in both Germany and America - the Morgenthaus who really made a name for themselves were Henry, who would serve as the American ambassador to Turkey under Woodrow Wilson, and Henry's son, Henry Jr., who was secretary of the treasury under Franklin D. Roosevelt. The senior Henry made his fortune through real estate investment. He was determined to repay his debt to his country through public service. In New York, he served on the Committee of Safety that investigated the notorious Triangle Shirtwaist sweatshop fire that took the lives of poor Jewish and Italian-American working women. An early supporter of Woodrow Wilson, Morgenthau was initially disappointed when the newly elected Democrat offered him the job of ambassador to Turkey. Morgenthau first turned it down, because he felt it had become a kind of Jewish slot the only diplomatic post to which a Jew can aspire", as he put it disapprovingly. But he changed his mind, and went on to play a key role in calling attention to the Turkish massacre of the Armenians: the century's first case of attempted genocide. Morgenthau also fought to protect the basic rights of other minorities, Christian and Jewish, who were also being persecuted in the Ottoman Empire. But his sympathy for the Jews who lived in Turkish Palestine did not extend to Zionism. In fact, he was an outspoken opponent of the idea of establishing a Jewish state: The Jews of France have found France to be their Zion, he wrote (prematurely) in 1921. The Jews of England have found England to be their Zion. We Jews of America have found America to be our Zion. His belief in assimilation, however, did not preclude a strong attachment to Judaism as a religion (he favored the Reform variety, with its emphasis on ethics rather than dogma, and he was also interested in Ethical Culture, Quakerism, and Christian Science). Henry and his wife had four children, but only one son, who became the apple of his father's eye. Henry Morgenthau Jr. was a shy youth, who grew up feeling a little overshadowed by his dynamic father. He was thoughtful and intelligent, but not a brilliant student. Despite encouragement in other directions, he opted to become a farmer in upstate New York, where he and his wife Elinor became close friends with Franklin and Eleanor Roosevelt. Born in America, Henry Jr. was even less concerned with being Jewish than his father. He had never even attended a Jewish seder (ceremonial meal celebrating the festival of Passover) until he was in his 50s. Yet, following a distinguished career as Roosevelt's secretary of the treasury (he was instrumental in financing the Allied war effort), he became deeply involved, first in assisting displaced persons after the war, and from that, in raising money for the newly established state of Israel. As secretary of the treasury, Henry Morgenthau Jr. organized the famous Bretton Woods conference, where the United States, Britain, the Soviet Union, and their allies, met to agree on the Anglo-American plan to stabilize postwar exchange rates via an International Monetary Fund and channel economic aid to war-torn nations via a World Bank. Less successfully, Morgenthau also came up with a controversial plan (known as the Morgenthau Plan) to make postwar Germany into an exclusively agrarian country to ins ure that it would never regain the military-industrial might to launch a third world war. The plan also included dividing Germany into zones of influence and thorough de-Nazification. It appealed to General Eisenhower, and intermittently to Churchill and Roosevelt, but was subsequently denounced by some critics as a species of "unchristianSemitic vengefulness." The fourth generation of Morgenthaus includes Robert, the respected Manhattan district attorney; Joan, whose coming out party was held at the Roosevelt White House; and Henry, the eldest child, who is a writer and television producer. Morgenthau draws on a variety of fascinating sources, including his great-grandfather's privately published - and vividly written - account of his early struggles, the private papers of his grandfather Henry, the voluminous diaries of Henry Jr., numerous interviews, and his own recollections. He fashions from these a brisk, cogent narrative that immerses readers in the drama of family history while explaining the large political and historical events along the way. His low-key, yet engaging style serves him equally well, whether he is dealing with international diplomacy, political maneuverings within the Democratic Party or the Roosevelt Cabinet, or tensions within the Morgenthau family. There is a slight tendency, as it moves forward in time, for the story to lose some of its narrative drive: Even a writer as clear-headed as Mr. Morgenthau can become a little overwhelmed by the increasing amounts of data that become available as the focus shifts from the distant past to the well-documented present. But overall, this is still one of the most lucid and captivating blends of history and family history that I've read in recent years.