The world remembers Einstein as a 'world citizen' because he believed in universal peace. When there was the rat race for becoming atomic power, he was worried about the aftermath of the bomb. He was really a world citizen who was concerned with humanity.
Einstein contribution to the world have been legendary and glorious. He made world aware of various unknown and unexplored areas in physics. One of the most famous paper in 1905 was about Einstein theory of relativity on the basis of which the famous formula describing the relation between mass and energy established. Einstein had rightly calculated in advance the extend from which light would be deflected from fixed stars through sun's gravitational field. His general theory of relativity provided a new interpretation to the world. He received noble prize for his valuable contribution in physics in 1921. Apart from this Einstein attempted to warn the people about the consequences of a atomic bomb. By writing a letter to the president, describing the consequences of such an action. After this dreadful incident of the bombing of Hiroshima and Nagasaki Einstein even proposed the formation of a world government but that didn't have any impact. Later he actively involved in politics and promoted peace and democracy and demanded an end to the arms build up