Defining a Work's "Historical Situation"
Because the "historical situation" always is enormously complex, the Marxist critics' first job is to convince readers that their particular set of socio-economic and historical facts are the most relevant to interpreting the text. As usual, the more facts within the work that a given set of material circumstances explains, the better the interpretation fits the work. Such an interpretation can be completely undone if the critics have to resort to "special pleading" to excuse elements of the text that do not fit the historical picture the critics are constructing. As is the case for any interpretation, it must account for relevant data of the work, it must explain them consistently, robustly, and without self-contradiction.
The first things a Marxist critic looks at when reading a modern work of literature are the back and front of the title page for the place and date of first publication.* That, and evidence of the author's historical situation when the work was being written, help "to situate" the work in the events and circumstances that produced it. [Early literature (e.g., Chaucer, Shakespeare, Behn) requires careful attention to the quality of the edition, but the author's historical circumstances are what would interest the Marxist.] Both Fitzgerald's The Great Gatsby and Hemingway's In Our Time were published in New York in 1925. They were written between the First and Second World War, in the third decade of the Twentieth Century but before the Stock Market Crash of 1929 signaled the beginning of The Great Depression. European economies tended to suffer significantly from the after-effects of the fighting on their soil, whereas the U.S. economy and physical infrastructure (factories, housing, etc.) was completely undamaged by the first war.
Gatsby's illicit fortune depends upon the "Volstead Act" which prohibited manufacture, sale or consumption of alcoholic beverages in the United States (the Eighteenth Amendment). From 1919 until 1933, the failure to enforce the act and its growing unpopularity with Americans led to a huge black market in illicit liquor--Jay Gatsby was a "drug dealer." Those circumstances are of the utmost importance in gauging the significance of his house, the parties, and the trip to New York City, but Fitzgerald could assume his audience would supply that knowledge to their interpretation of the text.. The Marxist also would be interested in Fitzgerald's writing as a form of myth-making about capitalism, consumerism, individualism, etc., even when the novel appears to be debunking the standard version of those myths (e.g., Jay Gatsby's idea of "self-improvement," Daisy's "green light" as Gatsby's self-manufactured, pathetically miniscule and misunderstood symbol of "the American Dream," etc).
Hemingway's characters in many of the italicized "interchapters" are or recently have been soldiers in World War I, their families, lovers, and friends. The war's lasting effects upon their minds forms a central historical circumstance that Hemingway did not have to explain to his audience because they would bring personal experiences or indirectly learned knowledge of the war to their interpretation of the text. The novel's depiction of the futility of the fighting, its savagery, its effect on European civilians, its false representation to American civilians, all would interest the Marxist as evidence of Hemingway's resistance to forms of "false consciousness," but they also would raise questions themselves about the historical realities underpinning the myths Hemingway constructs (e.g., "individualism," machismo and a fear of "effeminacy," the "code hero" cult, etc.).
* You also always should know the year a scholarly interpretation of literature was published before you attempt to read it. Very early Twentieth-Century works often may have serious defects in their critical methods. Mid- to late-Twentieth-Century work, if it follows New Criticism in a crude fashion, also may no longer be acceptable as a theoretically sound interpretation of the work. Simplifying applications of Structuralism, Feminism, and Psychoanalytic criticism from the late Twentieth Century also may be suspect. When in doubt, ask your instructor, but always be cautious when not using the most recent scholarship published in the best journals.