In this semi-autobiographical novel, an American named Roland Lancaster has a doomed affair with a younger woman, Elsa, in Cuba during World War II. The love story, in its happiest moments, parallels the idyllic life that author John Dos Passos had with his first wife, Katy.   The Great Days plots a key concern of the author's in the 1950s--America's rise to global prominence during World War II, and its loss of power in the years following the peace. In preparing the novel, Dos Passos studied James V. Forrestal, Secretary of Defense from 1947 to 1949. In his notes on the novel, he quotes Forrestal: "to achieve accommodation between the power we now possess, our reluctance to use it positively, the realistic necessity for such use, and our national ideals."