Conscription was introduced for all men between the ages of 21 and 30, and over two million men were sent to Europe.
In The Great Gatsby, both Gatsby and Nick have done war service in Europe. Gatsby tells Nick that he was promoted to major, and that each of the Allied governments gave him a medal. Nick says that he 'thoroughly enjoyed' his war experience.
However, many young men were shocked and disillusioned by their experiences in Europe, and Fitzgerald coined the term 'the lost generation' to refer to them.
Fitzgerald himself had volunteered to join the Army, but although he trained in America, his unit was never sent to Europe.