For one thing, World War I increased the reality and harsh imagery of poetry.  The poet who comes to mind most readily is Wilfred Owen with his gory details of how men died--read "Dulce est Decorum et" to get a more clear understanding.  Up until this time, war was considered glorious and patriotic--see Rupert Brooke's and Seigfred Sassoon's poetry to get a better understanding of this viewpoint. With the WWI poets, we no longer are "protected" from the ugliness, horror, and lies that were hidden from us prior to this time. 

World War I held a profound impact on English poetry and literature.  The questioning of moral structures and political orders, as well as the re-conceptualization of individual faith in structures that used to possess totality probably end up being the most profound influences on English poetry.  Poets and thinkers used the horrific nature of World War I to bring a sense of doubt and lack of totality to so much of human existence and consciousness, in general.  The faith in nationalism, nation building, imperialism, military strength, and patriotic identity were all brought into severe question through World War I, where death and destruction was wrought on so many levels.  At the same time, there was a genuine belief that the last of the carnage had not been experienced.  In my mind, Yeats' "The Second Coming" is probably the best example of the anguish and ceasless pain that was the legacy of World War I on European poets and English thought.

The First World War had a profound effect on English poetry. Many of the promising young writers and poets who went into the war as one sort of writer came out at the other end as a deeply different sort of writer. Poets such as Siegfried Sassoon, Rupert Brooke and Wilfred Owen had the tradition of the old English Romantic poets as models and examples. Some had led a blissful life of privilege in the soft lush greenery of England's shires where the only stress was leaving to go off to public boarding schools for rich families. So they had been writing about the delights of such a beautiful pastoral existence, giving thanks for it to God and country. They were prepared to fight and die to preserve it - or so they thought. When they found out the grisly and obscene horror of trench fighting, many began to write more protest poetry. Dulce et Decorum Est by Wilfred is a good example.