Eastman became a key figure in the left-leaning Greenwich Village community and lived in its influence for many years.
He combined this with his academic experience to explore varying interests, including literature, psychology and social reform.
In 1913, he became editor of America's leading socialist periodical, The Masses, a magazine that combined social philosophy with the arts.
Its contributors during his tenure included Sherwood Anderson, Louise Bryant, Floyd Dell, Amy Lowell, Mabel Dodge Luhan, Robert Minor, John Reed, Carl Sandburg, Upton Sinclair and Art Young.
That same year, Eastman published Enjoyment of Poetry, an examination of literary metaphor from a psychological point of view.
During this period, he also became a noted advocate of free love and birth control.

In his first editorial for The Masses, Eastman wrote:
This magazine is owned and published cooperatively by its editors.
It has no dividends to pay, and nobody is trying to make money out of it.
A revolutionary and not a reform magazine: a magazine with a sense of humour and no respect for the respectable: frank, arrogant, impertinent, searching for true causes: a magazine directed against rigidity and dogma wherever it is found: printing what is too naked or true for a money-making press: a magazine whose final policy is to do as it pleases and conciliate nobody, not even its readers.

The numerous denunciations of American participation in World War I published in The Masses, many written by Eastman, provoked controversy and reaction from authorities.
Eastman was twice indicted and stood trial under provisions of the Sedition Act, but he was acquitted each time.
In a July 1917 speech, he complained that the government's aggressive prosecutions of dissent meant that "[y]ou can't even collect your thoughts without getting arrested for unlawful assemblage".
 In 1918, The Masses was forced to close due to charges under the Espionage Act of 1917.

Eastman raised the money to send the radical John Reed to Russia in 1917.
His magazine published Reed's articles from Russia, later collected as Ten Days That Shook the World, his notable account of the Bolshevik Revolution.

Eastman had even delivered anti-war speeches on behalf of the People's Council of America for Democracy and the Terms of Peace.

In 1919, Eastman and his sister Crystal (who the next year was one of the founders of American Civil Liberties Union) created a similar publication titled The Liberator.
They published such writers as E. E. Cummings, John Dos Passos, Ernest Hemingway, Helen Keller, Claude McKay and Edmund Wilson.
In 1922, the magazine was taken over by the Workers Party of America after continuing financial troubles.
In 1924, The Liberator was merged with two other publications to create The Workers Monthly.
Eastman ended his association with the magazine.

In 1922, Eastman embarked on a fact-finding tour of the Soviet Union to learn about the Soviet enactment of Marxism.
He stayed for a year and nine months, observing the power struggles between Leon Trotsky and Joseph Stalin.
After attending the Party Congress of May 1924, he left Russia in June of that year.
He remained in Europe for the next three years.

Upon returning to the United States in 1927, Eastman published several works that were highly critical of the Stalinist system, beginning with "Since Lenin Died", which was written in 1925.
In that essay, he described Lenin's Testament, a copy of which Eastman had smuggled out of Russia.
In it, Vladimir Lenin proposed changes to the structure of the Soviet government, criticized the leading members of the Soviet leadership and suggested Stalin be removed from his position as General Secretary of the Soviet Communist Party.
The Soviet leadership denounced Eastman's account and used party discipline to force Trotsky, then still a member of the Politburo, to write an article denying Eastman's version of the events.
In other essays, Eastman described conditions for artists and political activists in Russia.
Such essays made Eastman unpopular with American leftists of the time.
In l
Eastman became a key figure in the left-leaning Greenwich Village community and lived in its influence for many years. He combined this with his academic experience to explore varying interests, including literature, psychology and social reform. In 1913, he became editor of America's leading socialist periodical, The Masses, a magazine that combined social philosophy with the arts. Its contributors during his tenure included Sherwood Anderson, Louise Bryant, Floyd Dell, Amy Lowell, Mabel Dodge Luhan, Robert Minor, John Reed, Carl Sandburg, Upton Sinclair and Art Young. That same year, Eastman published Enjoyment of Poetry, an examination of literary metaphor from a psychological point of view. During this period, he also became a noted advocate of free love and birth control.
In his first editorial for The Masses, Eastman wrote:
This magazine is owned and published cooperatively by its editors. It has no dividends to pay, and nobody is trying to make money out of it. A revolutionary and not a reform magazine: a magazine with a sense of humour and no respect for the respectable: frank, arrogant, impertinent, searching for true causes: a magazine directed against rigidity and dogma wherever it is found: printing what is too naked or true for a money-making press: a magazine whose final policy is to do as it pleases and conciliate nobody, not even its readers.
The numerous denunciations of American participation in World War I published in The Masses, many written by Eastman, provoked controversy and reaction from authorities. Eastman was twice indicted and stood trial under provisions of the Sedition Act, but he was acquitted each time. In a July 1917 speech, he complained that the government's aggressive prosecutions of dissent meant that "[y]ou can't even collect your thoughts without getting arrested for unlawful assemblage".  In 1918, The Masses was forced to close due to charges under the Espionage Act of 1917.
Eastman raised the money to send the radical John Reed to Russia in 1917. His magazine published Reed's articles from Russia, later collected as Ten Days That Shook the World, his notable account of the Bolshevik Revolution.
Eastman had even delivered anti-war speeches on behalf of the People's Council of America for Democracy and the Terms of Peace.
In 1919, Eastman and his sister Crystal (who the next year was one of the founders of American Civil Liberties Union) created a similar publication titled The Liberator. They published such writers as E. E. Cummings, John Dos Passos, Ernest Hemingway, Helen Keller, Claude McKay and Edmund Wilson. In 1922, the magazine was taken over by the Workers Party of America after continuing financial troubles. In 1924, The Liberator was merged with two other publications to create The Workers Monthly. Eastman ended his association with the magazine.
In 1922, Eastman embarked on a fact-finding tour of the Soviet Union to learn about the Soviet enactment of Marxism. He stayed for a year and nine months, observing the power struggles between Leon Trotsky and Joseph Stalin. After attending the Party Congress of May 1924, he left Russia in June of that year. He remained in Europe for the next three years.
Upon returning to the United States in 1927, Eastman published several works that were highly critical of the Stalinist system, beginning with "Since Lenin Died", which was written in 1925. In that essay, he described Lenin's Testament, a copy of which Eastman had smuggled out of Russia. In it, Vladimir Lenin proposed changes to the structure of the Soviet government, criticized the leading members of the Soviet leadership and suggested Stalin be removed from his position as General Secretary of the Soviet Communist Party. The Soviet leadership denounced Eastman's account and used party discipline to force Trotsky, then still a member of the Politburo, to write an article denying Eastman's version of the events. In other essays, Eastman described conditions for artists and political activists in Russia. Such essays made Eastman unpopular with American leftists of the time. In l
Max Eastman