Background: David Monroe Shoup (30 December 1904 - 13 January 1983) was a decorated general of the United States Marine Corps who was awarded the Medal of Honor in World War II, became the 22nd Commandant of the Marine Corps, and, after retiring, became one of the most prominent critics of the Vietnam War. Born in Indiana to an impoverished family, Shoup joined the military for financial reasons. Rising through the ranks in the interwar era, he was twice deployed to China during the Chinese Civil War. He served in Iceland at the beginning of U.S. involvement in World War II, and as a staff officer during the Pacific War.
Context: David Monroe Shoup was born on 30 December 1904 in Battle Ground, Indiana. His family lived on a farm in Ash Grove, but moved to Covington to live on a new farm in 1916. At age 12 he was enrolled in Covington High School, a competitive high school with an advanced curriculum. Shoup was an excellent student, maintaining high marks in French, English, physics, and history. Additionally, he was involved in several extracurricular activities, including basketball, and was class president in his senior year. He graduated in 1921. He later affectionately referred to his impoverished upbringing as that of an "Indiana plowboy." Regarded by friends as very sociable, he met Zola De Haven in his freshman year and later said he had been instantly attracted to her. They were both very competitive in academics and athletics, and the two dated throughout high school; they were married in 1931.  After high school, Shoup attended DePauw University where he was one of 100 awarded the Edward Rector Scholarship, giving him full tuition. Majoring in mathematics, he joined the Delta Upsilon fraternity and maintained high marks, narrowly failing the selection criteria for Phi Beta Kappa Society. He was on the track and field and rifle teams, and also competed in the wrestling and football teams. He won the Indiana and Kentucky Amateur Athletic Union marathon in 1925. He waited tables, washed dishes and worked in a cement factory to help pay his expenses. Lack of funds compelled him to take a year off after his junior year to teach school, and his expenses were further strained when he contracted a severe case of pneumonia and incurred hospital bills. He opted to enroll in the Reserve Officers Training Corps (ROTC) to offset his living expenses, and later recalled that this was the only reason he joined the military. He graduated from DePauw in 1926.  From an early age, Shoup was molded by the progressive ideas of Indiana politicians, sympathizing with rural progressives fighting against the interests of big businesses. He developed an anti-imperialist attitude, and his skepticism about American foreign policy, influenced by his small-town background, made him an outspoken opponent of the unnecessary use of military force. He felt the use of troops for economic or imperialist consideration was wrong, a viewpoint he would carry for his entire career.
Question: When did he join the military?

Answer:
after his junior year