Hello! You asked about modernism and why it is seen as a result of WWI. WWI ravaged Europe from 1914-1918. Why WWI brought about modernist thought in both art and literature:
1) The events of the war were a global horror; WWI plunged the world into chaos, violence and fear for the future as well as the present. The modernist influence on art and literature was a way to express those fears.
2) Modernism was a rebellion against the conventions of Romanticism; many felt that the conventions in traditional art and literature were inadequate to give voice to the despair of the masses after WWI. Wholesale slaughter and the dehumanization of industrialization detailed the sufferings of many. Modernist expression in both art and literature was a way to process those feelings through experimentation and individualism.
3) Modernist influence on art and literature expressed the death of the stability inherent in the Victorian era. Modernism was an expression of the new society; African Americans engaged themselves through the Harlem Renaissance, the feminist movement and the 19th Amendment gave women the right to vote in America, while Manet, Matisse and Picasso were the new avant-garde artists who rejected tradition in France. In America, a new group of writers known as the Lost Generation wrote novels filled with despair, realism, and brutal honesty.
So, you can see that modernism was a rejection of the cultural values of traditional Victorian society. The writers and artists simply felt that the old ways of expression were inadequate; yet, they may not have realized that the old values were firmly destroyed by radical industrialization and war. Materialism made no room for the traditional qualitative values of courtesy, compassion or respect. Although capitalism itself brought many new benefits, it was largely exploited by the powerful few at the expense of many.
Some links you may find helpful in your research on modernism and WWI are below. Thanks for the question!