While attending Temple College, Claude tried to convince his parents that attending the State University would give him a better education. His parents ignore his pleas and Claude continues at the Christian college. After a football game, Claude meets and befriends the Erlich family, quickly adapting his own world perception to the Erlichs' love of music, free-thinking, and debate. His career at university and his friendship with the Erlichs are dramatically interrupted, however, when his father expands the family farm and Claude is obligated to leave university and operate part of the family farm.
Once pinned to the farm, Claude marries Enid Royce, a childhood friend. His notions of love and marriage are quickly devastated when it becomes apparent that Enid is more interested in political activism and Christian missionary work than she is in loving and caring for Claude. When Enid departs for China to care for her missionary sister, who has suddenly fallen ill, Claude moves back to his family's farm. As World War I begins in Europe, the family is fixated on every development from overseas. When the United States decides to enter the war, Claude enlists in the US Army.
Finally believing he has found a purpose in life - beyond the drudgery of farming and marriage - Claude revels in his freedom and new responsibilities. Despite an influenza epidemic and the continuing hardships of the battlefield, Claude Wheeler nonetheless has never felt as though he has mattered more. His pursuit of vague notions of purpose and principle culminates in a ferocious front-line encounter with an overwhelming German onslaught.

Question: What prompts Claude become a soldier?
Answer: The United States enters World War I.
Alternative answer: World War I erupts.