Fame was not a natural fit for Einstein's shy personality. He soon tired of being stopped on the street by complete strangers, sometimes telling them, "Very sorry, I am often mistaken for the famous Professor Einstein." But such evasions could not reduce the magnitude of Einstein's celebrity. From the 1920s through the 1950s, much of the public remained convinced that Einstein was not only one of the world's smartest individuals, but one of its wisest as well. People constantly sought his advice on matters great and small. (A teenage girl once wrote him asking for help with her math homework; Einstein replied, "Do not worry about your difficulties in mathematics; I can assure you that mine are much greater.") Uncomfortable as he was in the spotlight, Einstein resolved to use the platform offered by his fame to speak out on the major political and humanitarian problems of his time. Albert Einstein became a forceful advocate for the causes dearest to his social vision—internationalism, pacifism, and Zionism.
The three causes were completely interlinked in Einstein's mind, and his belief in all three was rooted in his own life experience as a German Jew who lived through two world wars and one Nazi reign of terror. The expansionistic and militant wave of nationalism that swept over Germany through much of the early twentieth century instigated the two worst armed conflicts in human history, World Wars I and II. That same wave of German nationalism repeatedly threw Albert Einstein's life into chaos.
In 1896, when he was just 17 years old, Einstein—then living in Switzerland—had to renounce his German citizenship to avoid mandatory service in the Kaiser's military. By the time the First World War broke out in 1914, Einstein was back in Germany, serving as director of the Kaiser Wilhelm Institute for Physics in Berlin. Einstein was appalled by the hypernationalism that engulfed even Germany's scientific community; when 93 eminent German scientists published a jingoistic "Manifesto to the civilized world" backing the country's war effort, Einstein responded by drafting a pacifistic and anti-nationalistic "Manifesto to the Europeans," which argued—very much against the spirit of the times—that "the struggle raging today can scarcely yield a 'victor'; all nations that participate in it will, in all likelihood, pay an exceedingly high price." Einstein could find only two others to co-sign his treatise, and many of his Prussian colleagues scorned him for his lack of patriotism in a time of war.
But defeat in the Great War seemed to dampen the flame of German nationalism after 1918, and Einstein even re-applied for German citizenship in 1920 in a gesture of solidarity with the liberal new government of the Weimar Republic. But Weimar collapsed amidst the economic ruin of the late 1920s and 1930s, and a menacing new manifestation of German nationalism appeared on the political scene—Nazism.
As a Jew, Einstein quickly found that his position as the most renowned scientist within a society overtaken by anti-Semitic madness had become untenable. Nazi scientists began to attack Einstein's theories as "Jewish physics," proposing alternative "Aryan physics" instead and blacklisting professors who continued to teach using Einstein's concepts. In 1932, just before Hitler fully consolidated his hold on power in Germany, Einstein and his family left for the United States, never to return. In 1940, for the second time in his life, Albert Einstein renounced his German nationality and became a citizen of the United States. He would never forget or forgive the German people for their participation in Hitler's crimes against humanity.