What was the impact of war on Modernism?
Arguably, the Modernist era began after the First World War. Every aspect of society changed. First, industry developed, particularly with the introduction of the assembly line in the United States. This allowed women to work, which expanded their economic power and brought in a new era of consumerism and -- for better or worse -- advertising. Women also gained the right to vote in the United States in 1920 and, in Britain, full suffrage was granted in 1928.
Social mores changed. Strict propriety was demanded, particularly of the upper- and middle-classes in the Victorian and Edwardian eras. It did not matter, really, if one engaged in "immoral" acts privately, as long as they were not found out. This hypocrisy did not survive well into the twentieth century without being questioned or criticized.
There was something about the First World War that was especially cataclysmic to the human spirit. It raised existential questions: Was there any certifiable meaning to life? Is there any such thing as the "truth" of existence? The answers from artists and writers of the period was "no" to both questions. Morality became an individual thing. In his novels and short stories, Hemingway promotes the idea of every man (in some ways, he was rather chauvinist) having his own moral code. Suicide is no longer regarded as an unspeakable sin but a real consideration in the face of existential crises. This is something that Virginia Woolf explores in Mrs. Dalloway. Finally, sexuality is no longer unspeakable, it is celebrated in Ulysses; and aspects of female autonomy, such as the right to an abortion, are addressed in Dubliners.
In visual art, Cubism is probably the most important movement. Though Georges Braque and Francis Picabia should not be neglected from any discussion, Picasso is clearly the most important. Picasso presents figures that are distorted and dismembered. They are flat shapes in the canvas, illuminated only by color. Depending on the period in which we regard Picasso's work, that color could have been either red or blue.
His paintings appear to be a comment on what it meant to be human at this time. The notion of being "modern," as wonderful as it sounds in some respects, is also disorienting. There is no longer a clear sense of right and wrong, no longer any rules on what is to be done and what is not to be done to survive in the world. Much of this is due to what people learned in the aftermath of the First World War: that death was far closer than any of us had realized. It was, therefore, important to live an honest and meaningful life.