At the time of World War I, Einstein is already being kept under surveillence because of his pacifist stance. Even during the Weimar Republic, he is being observed by the state because of his prominent political role.
After Einstein's emigration to the United States, the FBI compile an extensive file on him. His public opinions on questions dealing with global politics and his commitment to Americal civil rights cause him to appear suspicious in the McCarthy era. The authorities receive numerous letters from fervent informers that present Einstein as a danger to national security due to at times dubious reasons. The FBI boss, Edgar J. Hoover, is obsessed in trying to convict Einstein as a communist with "unamerican attitudes."
However, Einstein is too famous. The FBI do not risk taking action against the prominent scientist and cosmopolitan. It also must be acknowledged that the compiled material can hardly be used as a document of subversive activities. It is too unbelievable and absurd.