The post-World-War-I Zeitgeist is usually identified as one of disillusionment and pessimism. An entire generation of young men had gone to war with expectations of glory, and had seen only death and destruction on a massive scale, beyond that of any conflict in history. If there was a special reason...
other than the scale of casualties for this despairing attitude, it may be the fact that the war accomplished nothing. Millions of men had died not to satisfy an idealistic goal (such as, for instance, the abolition of slavery in the US Civil War) but merely to satisfy the power-hungry, imperialistic ambitions of the ruling class of Europe.
Eliot's "The Hollow Men" conveys the idea that those who survive—not specifically war veterans necessarily, but people in general in the modern age—are the living dead. "Death's other Kingdom" would appear to be the actual land of the dead, as opposed to the land of living death which earth has become. The dead look back upon the living and see the survivors "not as lost / Violent souls, but only / As the hollow men / The stuffed men." The implication is that men are now incapable of feeling, of experiencing the emotions that animated people in the past. The living—or is it the actual dead, for we cannot be sure?—wander blindly: "We grope together / And avoid speech / Gathered on this beach of the tumid river." As in the Gertrude Stein quote that Hemingway uses as the epigraph to The Sun also Rises, "You are all a lost generation," humanity is lost, disconnected from the things that gave meaning to life before the war blew everything to bits.
But is it the war that did this? If we look at Eliot's most famous poem, "The Love Song of J. Alfred Prufrock," which was written before the war, we see similar themes, though expressed in a less stark, less complete way than in "The Hollow Men." The thinking of artists and intellectuals before 1914 had already taken a pessimistic turn, often interpreted as a reaction against the optimism of the Victorian age; and the aesthetic of the nineteenth century, with its sweeping emotionalism, had begun to seem outdated and even embarrassing to the modern mind. One might ask if "The Hollow Men" expresses something not necessarily connected at all with the time and place in which it was written and encapsulates sentiments that are universal and have been at the core of at least one dimension of human thought throughout all of history.