Because Regeneration is a novel that focuses on the First World War, it explores many of the themes common to literature written during and following the war, including the cause and effects of war, the limits of ideologies like nationalism and masculinity, and both the medical and popular reactions to the psychological traumas created in the war. Critics have treated each of these extensively. Moreover, because much of Barker's earlier work was historical fiction about women, critics often comment on her treatment of women in the novel.
The novel extensively focuses on the effects of losses during wartime. As The Guardian noted when discussing her awards for The Ghost Road, the series gave her the reputation as "The woman who understood war". Barker stated in an interview with Wera Reusch that "The trilogy is trying to tell something about the parts of war that don't get into the official accounts". She goes on to state that "One of the things that impresses me is that two things happen to soldiers in war: a) they get killed or b) they come back more or less alright. It's really focusing on the people who do come back but don't come back alright, they are either physically disabled or mentally traumatised."
One of the focuses of the novel is on how combatants perceive their experiences. In her article discussing the novel's representation of death, literary critic Patricia E. Johnson describes how contemporary society tends to make the casualties and experience of war more abstract, making it hard for non-combatants to imagine the losses. Johnson argues that the entire Regeneration Trilogy breaks the boundaries created by modern society's abstraction of war and its casualties because "mutilation and death are re-presented in a ways that escape warfare's typical conceptual categories, thus ..."realising" modern warfare by reconnecting language and material substance." In discussing the first novel specifically, Johnson highlights how the book "repeatedly employs synecdoche" to emphasise the visceral experiences, by describing eviscerated human flesh and how the characters respond to those experiences. She describes experiences like Burns's horrifying head first disembowelment of a corpse as allowing the readers to understand two things: first, that memories of the combatants are recorded in terms of their relationship to actual people, rather than in the vague ideas of people represented by war memorials; and second, the conceptual opposition in Western culture between flesh or body parts and the social definition of a person (for further discussion of this philosophical issue see Mind-body problem).
Much of the novel explores the types of cultural ideologies, like nationalism and masculinity, that facilitated the War. Barker states that she chose to write about World War I "because it's come to stand in for other wars, as a sort of idealism of the young people in August 1914 in Germany and in England. They really felt this was the start of a better world. And the disillusionment, the horror and the pain followed that. I think because of that it's come to stand for the pain of all wars." Critic Kaley Joyes argues that choices like the inclusion of the work by poet Wilfred Owen in the novel, whose life has been romanticised as "an expressive exemplar of the war's tragic losses," highlights this thematic interest in breaking down the common ideological interpretations of the war.
The tension between traditional models of masculinity and the experiences within the war runs throughout the novel. Critic Greg Harris identifies Regeneration, along with the other two novels in the trilogy, as profiling the non-fictional experience of Sassoon and other soldiers who must deal with ideas of masculnity. These characters feel conflicted by a model of masculinity common to Britain during this time: honour, bravery, mental strength, and confidence were privileged "manly" characteristics. Yet they explore, internally and through conversation, what that model means for them and how the war changes how they should experience it. In an interview with Barker in Contemporary Literature, Rob Nixon distinguishes between these ideas of "manliness" and the concept of masculinity as providing a larger definition for identity. Barker agrees with his assessment, saying, "and what's so nice about them is that they use it so unself-consciously: they must have been the last generation of men who could talk about manliness without going "ugh" inside."
In his discussion of the novel, Harris describes this "manliness" as becoming, for Barker's characters, an "unrealistic militaristic-masculine ideals"; practices such as the deliberate repression of emotion consume the novel's characters and create psychological instability, as well as being the cause of extensive discrimination during the war. Harris highlights how this thematic treatment fairly represents how the question of masculine identity effected Sassoon and other shell-shocked WWI soldiers. Harris also describes Barker, as author, and Rivers, as a period innovator, demonstrating how the use of therapy on soldiers offers an opportunity to shape and rethink this model of masculinity. The idea of reintegrating emotions, in relation to questions about the nature of masculinity, are an important part of the novel; Barker focuses on the same type of emotional reintegration that historians have identified in River's actual methods for treating victims of the war.
Psychology and trauma
The novel's use of a mental hospital as the main setting, along with psychologist Rivers' treatments of soldiers and their war trauma, focuses much of the novel on the psychological effects of war. In doing so, the novel follows in the tradition of novels like The Return of the Soldier (1918) and Mrs. Dalloway (1925). Many critics focus on this interest in the effects of trauma. For instance, Ankhi Mukherjee describes the failure of characters to turn their memories into a narrative through the medium of talk therapy. Mukherjee describes River's approach to therapy as "autogenesis," or self-understanding through structuring their reaction to traumatic experiences.
Sigmund Freud is an important influence on the novel's approach to psychology, and this influence has roots in the historical context of the novel, because Rivers was influenced by the writings of Freud on neurosis and Sassoon wrote about the experience of Freudian psychoanalysis in his Sherston's Progress. While Rivers disagreed that neurosis was due to sexual factors he considered Freud's work to be of "direct practical use in diagnosis and treatment". Craiglockhart's approach to compassionate talk therapy had roots in the Freudian treatments of hysteria, using tools such as expressing compassionate understanding for patients and helping patients interpret dreams.
Women and the domestic
Some critics have focused extensively on the place of women within the novel. Critical interest in women partially is a reaction to Barker's previous novels focusing on working class women's history, and this novels departure from this topic to focus on men. In her companion to the novel, Karin Westman situates the novel as a response by Barker to the critical reception that had slotted her into particular literary roles, such as a woman writer only interested in writing about women. However, Baker has repeatedly talked about the novels as connecting to her thematic interests in her earlier works, like feminism and women. Barker describes the novel as providing a voice for the homefront, stating that "In a lot of books about war by men the women are totally silenced. The men go off and fight and the women stay at home and cry; basically, this is the typical feature. And the women in the trilogy are always deeply significant, and whatever they say in whatever language they say it in, it is always meant to be listened to very carefully." In particular, Barker is interested in the contradictions placed on women's expectations during war period, and its history; for example, she points out that the women in the munitions factories were expected to produce weapons to kill thousands, but a woman who attempts to abort her unborn child is criticised.
Part of writing female perspectives into war fiction is to provide a larger sense of the domestic repercussions. Critic Ronald Paul notes that Regeneration and its sequels are some of the first canonical novels since Rebecca's West's The Return of the Soldier or Virginia Woolf's Mrs. Dalloway that deal with the repercussions of the war but whose author wasn't a male soldier during the war. Paul describes such novels which deal explicitly with domestic effects of shell shock as part of Barker's self-described "very much female view of war".