The setting of Virginia Woolf's Mrs. Dalloway takes place in London, during the Summer of 1923. This time frame sets the novel approximately six years after the end of World War I which, in itself, was an event that set off a transforming paradigm shift across society. As it is often the case in post War societies, the roles of those who compose it change as their importance is questioned.
In post-War London, particularly in the area of Westminster where the novel develops, the Dalloways would have represented the stratum of high society that showed itself to the people as unscathed from the horrible effects of war.
The War was over, except for some one like Mrs. Foxcroft at the Embassy last night eating her heart out because that nice boy was killed and now the old Manor House must go to a cousin; or Lady Bexborough who opened a bazaar, they said, with the telegram in her hand, John, her favourite, killed; but it was over; thank Heaven — over.
Regardless of the fact that a major world conflict has taken place not long before the novel begins, the upper classes of London's fashionable set stick stubbornly to their established social dynamics, choosing to be oblivious to what "the others" feel. For this reason, Woolf integrates the character of Septimus as a way to create contrast between those who intend to move forward, and live life as they know it, and those who are too unfortunate to be able to do so.
Septimus comes in as a contrasting and somewhat existential twin of the character of Mrs. Dalloway. They never meet in the story, but his experiences metaphysically affect Mrs. Dalloway and her questions about life, choices, and fate.
A soldier who witnessed the death of his comrade, Septimus is suffering from an obvious case of severe Post Traumatic Stress Disorder (PTSD). As a result of the ignorance for his condition, there is an overall inability to analyze the extent of his illness. This is what leads him to end his life violently.However, it is the sum of Septimus's observations about people, life, and death what makes up for the meat of the novel; his partial insanity helps him see life the way that it really is.
Conclusively, the importance of War in the novel is that it serves as the conduit through which the character of Septimus develops philosophically and psychologically. His traumatic emotions and constant reminders of death are juxtaposed to Mrs. Dalloway's questions about the meaning of life. However, when Mrs. Dalloway hears about his death she finally recognizes the reality of it, and therefore the importance of valuing life the way it comes our way, regardless of what we expect it to be like. Therefore, war is the agent of change in the main characters of the story.