The author gave his novel the title An American Tragedy in order to emphasize the idea that the tragedy could only have happened in America--or at least that it was far more likely to occur in America than anywhere else in the world. In other parts of the world a young man like Clyde Griffiths would have been far less mobile. In Theodore Dreiser's novel the protagonist travels all the way across the continent from San Francisco to Lycurgus, New York to meet his fate. In any other part of the world a young woman like Roberta Alden who grew up on a little farm would have been unlikely to leave the farm and travel to a strange town by herself. America was becoming industrialized. There was a demand for piece-workers in the manufacture of all kinds of goods. Many of the workers were women because they were skillful at routine, precision work such as Roberta performed in the mass production of shirt collars. (Dreiser's heroine Carrie Meeber was attracted from the farm to Chicago by the opportunities for women in light manufacturing jobs, often called sweatshops.) Also, women always worked for lower wages than men. Elsewhere in the world a young man like Clyde would have been less likely to meet a girl like Roberta, because such lonely country girls would not have been available in such numbers. It was the industrialization of America that brought Clyde and Roberta together. And elsewhere in the world a lower-class youth like Clyde would have found it nearly impossible to become acquainted and then in love with an upper-class girl like Sondra Finchley, because the barriers between social classes were to firmly established in older societies. In any other part of the world Clyde could not have aspired to become a member of the elite social class married to a rich girl and socializing with the kinds of people whose leisure activities were always being written up in the Society sections of local newspapers. It is noteworthy that Clyde is finally executed by the new American invention, the electric chair.
According to an article on An American Tragedy in Wikipedia:
Dreiser based the book on a notorious criminal case. On July 11, 1906, resort owners found an overturned boat and the body of 20-year-old Grace Brown at Big Moose Lake in the Adirondack Mountains of Upstate New York. Chester Gillette was put on trial and convicted of killing Brown, though he claimed that her death was a suicide. Gillette was executed by electric chair on March 30, 1908.
Truman Capote's best-known book In Cold Blood seems to have been influenced by Dreiser's masterpiece An American Tragedy, although Capote's novel uses real names, places, and events, while Dreiser's is heavily fictionalized.
The title "An American Tragedy" is a play on the expression "the American dream." That dream is supposedly to rise from rags to riches, or at least to do better than our parents did before us.
In the book, Clyde Griffiths dreams of living a better life. His parents are poor evangelists who shun material wealth. Clyde wants that wealth and everything that goes with it. He tries to get it through working, but he finds an easier way to attain it: marry a wealthy woman. The tragedy is that his dream is a nightmare for those who love him. Clyde will stop at nothing, not even murder, to get what he wants, and he ends up losing everything.
FYI, the book was made into a movie, but the title was changed to "A Place in the Sun." It's an excellent movie.