He was legally German until 1933, when he renounced his citizenship. Then he applied for an American citizenship, what made him American until his death. A nationality is defined by a commitment to a nation and may be chosen by the individual according to the human right of free association (see United Nations Declaration of Rights). At the point Albert Einstein had officially renounced his German citizenship, despite he has always been carying it half-heartily only, he had declared he is not a German, and by adopting an American citizenship, he also declared he is American. Thus we have to respect his decision and write in our history books he was American.