One of the major precursors to the modernist movement was Charles Darwin. In the mid-nineteenth century, he began developing his theories of evolution. Over the course of fifty years, there were major technological developments that allowed humans to see farther and smaller than ever before, and near the turn of the century, radioactivity was discovered. Slowly but surely, humans and their mastery of reason began to look less and less attractive, beholden to the whims of biology and physics, vessels lost in the great cosmos. Where there was once faith in God and our ability to choose objectively "good" options for ourselves, science began to make humans look insignificant.

In concert with this idea was the development of the field of psychology. Wundt showed how easy it is to condition animals to believe certain thing and behave in certain ways. Only shortly after, Freud took this idea further, applying it to humans, and developing theories of development that suggest we have little control over our behaviors and beliefs. Instead, humans are born into certain predictable patterns and conditioned to act in certain ways, ultimately torn between constant guilt and sexual gratification.

In this way, the invention of new disciplines contributed greatly to a general questioning of humanity's place in the world. Instead of God's chosen, rational children, humans lost a sense of primacy. The horrors of the World Wars, which could easily be attributed to science's capabilities and dark subconscious drives within humans, simply amplified the darkness that undergirded much of modernist art.

Modernism has been arguably the most important artistic movement of the twentieth century. It is characterized by a certain malaise, a generally dark perspective on the state of the world, a sense of the meaninglessness of what were once held as society's certainties, and a conviction that only the rebellious artist tells the truth about the world.

In many works, there is a sense of disillusionment, alienation, and meaninglessness as a result of World War I:

The coming of World War I fulfilled the modernist predictions of a coming fragmentation and destruction beyond anything they could have imagined. [eNotes]

Other major events which affected Modernism are the Great Depression, the rise of Fascism, and the horrors of total war with World War II.

In American literature, Ernest Hemingway became a voice of the alienated and disillusioned, particularly through his male characters. In one story, "Another Country," soldiers are wounded and thus removed from battle. After becoming distanced from the war, they find themselves strangely alienated and disillusioned with the purported "heroism" of being a soldier.

In British literature, the Irish Literary Revival, also known as the Celtic Renaissance, began in late Victorian times. It sought to revive the dying Gaelic language, to explore Celtic history and language, and to express the Irish spirit. Writers in Ireland and in many former British colonies have taken a searching look at the legacy of empire.

Many of the poets who survived the First World War went on to express the perspectives of the "lost generation" in the Modernist poetry of the 1920s. Prominent among the Modernist poets was T. S. Eliot, whose long poem The Wasteland stands as a monument to the bitterness and despair of this "lost generation." At times poets made direct statements of social protest as well. For instance, W. H. Auden wrote in his famous poem, "September 1, 1939":

I sit in one of the dives
On Fifty-second Street
Uncertain and afraid
As the clever hopes expire
Of a low dishonest decade.

Despite the rise of unromantic Modernism, a romantic strain remained in British poetry through the work of such poets as Dylan Thomas.

The class system and the plight of the industrial worker emerged as key themes in the works of D. H. Lawrence. Irish writer James Joyce also wrote of working-class life in such works as The Dubliners.

Many social and political issues in the 1930s and 1940s gained attention and brought about the emergence of a new group of novelists. One of these novelists was Aldous Huxley, who in Brave New World satirized the misuse of science. Another topic of satire was totalitarian governments as in George Orwell's fable Animal Farm.

The literary period of Modernism (1900-1945) followed the Realist movement. This period was not the typical literary movement (signified by a somewhat universal ideology of the writers of the period). Instead, the Modernist movement existed as numerous small groups of writers. Historically, the most significant event during this literary period was that of World War I. In a sense, the literature of the period mirrored the conflict, division, and destruction of the war. Writers of this period tended to speak out against conformity (acting as literary rebels). 

Human events which fueled the movement were also ground in the war. Prior to WWI, literary texts illustrated the breaking away from conformity (as seen in James Joyce's Ulysses), while, after the war, texts illustrated the reality behind destruction (as seen in T.S. Eliot's "The Waste Land").

The themes of the period placed value on the human experience. Alienation, the past, and the stream of consciousness all played a part in many Modernist texts. Given that the period led America into the human rights era, it is of no surprise that Modernist texts forced readers to examine life from a very different perspective. Modernists relied upon the flow of thoughts (stream of consciousness) which led readers through the texts. This mode of narration illustrated more than one's literal experience in life. Instead, it illustrated how people thought about things such as conformity, war, violence, destruction, and change.