At midnight, December 31, 1925, citizens of the newly proclaimed Turkish Republic celebrated the New Year. For the first time ever, they had agreed to use a nationally unified calendar and clock.

Yet in Istanbul--an ancient crossroads and Turkey's largest city--people were looking toward an uncertain future. Never purely Turkish, Istanbul was home to generations of Greeks, Armenians, and Jews, as well as Muslims. It welcomed White Russian nobles ousted by the Russian Revolution, Bolshevik assassins on the trail of the exiled Leon Trotsky, German professors, British diplomats, and American...