Albert Einstein’s impact on war:

Einstein began speaking out against war and violence before World War I, but after experiments during a solar eclipse proved general relativity in 1919 (“Einstein Theory Triumphs,” said a sub-headline on the story in The New York Times), he became an overnight star.
As the “winner in this contest with Newton,” Schulmann said, Einstein was a media sensation. Front-page headlines followed him across America when he arrived for a tour in 1921, and his pronouncements on peace were already leading to his being seen as the “conscience of the world.”
Once he was compelled to abandon Germany during the rise of Hitler, Einstein emerged as a leading symbol of pacifism in the 20th century, held by some thinkers on a par with Mohandes K. Gandhi and Albert Schweitzer.
But Einstein’s commitment to pacifism was never absolute, as was publicly thought. In 1939, six months after the discovery of uranium fission, he began work on a letter urging President Franklin D. Roosevelt to build the atomic bomb, for fear that Germany would get there first. “In view of this situation, you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America,” he wrote in August, going on to recommend specific sources of uranium ore in the former Czechoslovakia, Canada and the Belgian Congo.
Years later he called the entreaty the “one great mistake in my life” and said in a letter to President Harry S. Truman: “I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.”

In a letter exchange with Sigmund Freud, Einstein expresses the following thoughts:
The only possible solution Einstein sees, is for the nations of the world to create a legislative and legal body that will be called upon in all matters of conflict that may arise between them. A sort of Supreme Court of all nations. All nations would agree to call upon this court when conflict arises and to follow the decisions and directives of this court. Of course, he states, that this is an institution created by people. Such a court will be all the more prone to influences from outside the court if its own power is insufficient to enforce its decisions in practice. The decisions of a court will be closest to the ideas of justice within the society for which it acts, the more power this society can invest in enforcing respect for those ideas. "We are far from creating an organization with sufficient power to enforce the laws it decrees", Einstein says. His first conclusion is, that it will be necessary for nations to give up a certain amount of their sovereignty. It is without doubt the only way to security.
So far, all attempts in this direction during the last decades have failed. Obviously strong psychological mechanisms with the human psyche are working against these attempts. Some of these mechanisms can be identified. The minority in power, within any given society, will resist any infringements upon its power. Einstein notes that this striving for power is driven by materialistic and economic wishes. He refers to the minority within any society which will stop at nothing to gain advantages for itself and will not stop at war or weapons deals in order to increase its own power and influence.
As their conversation continues, the question arises, "why does the majority allow itself to be used by the minority in power?"
The minority stands to gain and the majority stands to suffer and to lose. Einstein states that the minority in power rules over the schools and the press, and also has influence over the religious organisations. The minority in power uses these institutions to manipulate and channel the feelings of the masses in order to use people for their own gain.
This however, he claims, cannot be the only reason that the majority lets itself be used in these ways, and will indeed let itself be driven to the extent of frenzy and self-sacrifice. Einstein concludes that there must be a force within humans, a wish to hate and destroy. A force that during normal times is dormant, only showing itself in the abnormal. It can however, easily be awakened, and increased to the extent of mass-psychosis.

Ref: Albert Einstein: Why War? and The Complete Works of Sigmund Freud. www.wikipedia.org/wiki/Einstein