On Pacifism
www.th.physik.uni-frankfurt.de/~jr/phys.einstein
"Why should one not admit a man [to the United States]...who dares to oppose every war except the inevitable one with his own wife?" (1932) (1)
Albert Einstein viewed the militarism with his homeland as disgusting. At the age of 16 renounced his German citizenship and became a Swiss citizen vowing never to hold German citizenship again. During World War I Einstein was the co-founder of the 'Bund Neues Vaterland'(League of the New Fatherland). "Its direct aim was to bring about an early and just peace, but it also had the long term objective: the establishment of an international organization which would make future wars impossible."(3) The work of this group was eventually banned and an underground operation continues until it could be re-founded a few months before the end of the war.
The League of Nations had a link to Einstein in its Committee of Intellectual Cooperation, also considered the ancestor of the UNESCO. His involvement on this committee was short lived due to the prejudices against his German and Jewish background. Einstein also saw that the group catered to the powerful and wealthy.
When Adolph Hitler gained power in 1933, Einstein changed his opinion on militarism. He started to tell the nations of Europe that military preparedness is what should be happening in their countries. He realized that the only way to stop Hitler was to meet force with force. (I think he was thinking of Newton's 3rd law which is for every action there is an equal and opposite reaction. Unfortunately, the opposite reaction was appeasement for the nations of Europe.)
Luckily for Einstein he moved to the United States of America before he could become one of murdered people in Nazi Germany. Einstein also used his influence to try and rescue other people from the wrath of Hitler.
Einstein used his influence to write a letter to the President of the United States, then Franklin D. Roosevelt, encouraging the USA to start developing an atomic bomb. He wrote this letter with the impression that the Germans were also developing a bomb. The development of a Nazi atomic bomb never happened.