This is a huge question that can't be adequately dealt with short of a book length treatise, but an outline of a few of the effects of the war can be touched on. First, World War I was so obviously a huge mistake and a wasteful bloodbath that left European...
This is a huge question that can't be adequately dealt with short of a book length treatise, but an outline of a few of the effects of the war can be touched on. First, World War I was so obviously a huge mistake and a wasteful bloodbath that left European and American philosophy reeling. Nineteenth-century optimism and belief in progress had been undergirded by enlightenment notions of philosophic rationalism, a belief that humankind could gather the correct data and make decisions that would work in favor of making the world an ever-better place to live.
World War I punched a hole in the idea of rationalism. As a result, philosophical or related systems that took into account the dark irrationality of the human psyche gained traction, such as Freud's theories of the unconscious. These were, strictly speaking, psychology, but they had a huge impact on early- to mid-twentieth century philosophical thought. Philosophical nihilism gained more traction: nihilism rejects value-based thinking and offers a pessimistic view of human existence, while being skeptical that "truth" can ever be known or even has reality.
In Germany, the shock and humiliation of losing World War I sent philosophy into its own dark spaces. Right-wing philosophies of "blood and soil" and the mystical worldview of the Germanic spirit challenged enlightenment (especially French) liberalism and ideas of universal brotherhood to embrace ideas of a special world destiny for the Aryan race.
Nietzsche is an important post-war figure. Looking at the ruin and carnage wrought by a world war, European intellectuals began to embrace the idea that perhaps humans are caught, as Nietzsche contended, in a prison house of language and perception that lead us into dark places. This idea began to overshadow the idea that language and perception were a clear windowpane leading to truth.
We are still living in the aftermath of a post-World War I shaking up of confidence in the power of humankind's rationalism. As philosophy since Socrates and Plato has been based on rationalism, the entire field of philosophy has been shaken up .