While he is often quoted by freethinkers, how many are aware of the fact that Mark Twain was a vehement critic of war and U.S. imperialism?
From 1901 until his death in 1910, Twain was the vice president of the American Anti-Imperialist League, founded in 1898. Twain and his associates, which included John Dewey and William James, sought to rally opposition to the annexation of Cuba, Puerto Rico, Guam and, in particular, the Philippines.
Albert Einstein was an active anti-imperialist as well. Einstein participated in the League against Imperialism which criticized U.S. imperialist policy in Latin America, among other subjects. Einstein was actively committed to bringing an end to war.
In September 1932 Sigmund Freud responded to Albert Einstein's letters addressing the bane of human warfare. In the letter, "Why War?", Freud explained that he and Einstein "rebel so violently against war" "because everyone has a right to his own life, because war puts an end to human lives that are full of hope, because it brings individual men into humiliating situations, because it compels them against their will to murder other men, and because it destroys precious material objects which have been produced by the labors of humanity." In short, Freud wrote: "we simply cannot any longer put up with (war)."