During what war was the novel "The African Queen" set?

The African Queen is a 1935 novel written by English author C. S. Forester. It was adapted to the 1951 film with the same name. The story opens in August/September 1914. Rose Sayer, a 33-year-old Englishwoman, is the companion and housekeeper of her brother Samuel, an Anglican missionary in German-colonized Tanganyika (present-day Tanzania). World War I has recently begun, and the German military commander of the area has conscripted all the natives; the village is deserted, and only Rose and her dying brother remain. Samuel dies during the night and Rose is alone. That day, another man arrives at the village: this is a Cockney named Allnutt, who is the mechanic and skipper of the African Queen, a steam-powered launch, owned by a Belgian mining corporation, that plies the upper reaches of the Ulanga River. Allnutt's two-man crew has deserted him at the rumors of war and conscription. Allnutt buries Samuel Sayer and takes Rose back to the African Queen, where they consider what they should do.

Allnutt is changed to a Canadian in the film to explain star Humphrey Bogart's accent. In the film the German captain attempts to hang Rose and Charlie. They are married by the captain before the execution.