Truman left high school to help out his family. He worked at odd jobs before moving back to his father's farm, where he worked until he was called to fight in World War I. Truman had joined the Missouri National Guard in 1905. He helped organize the Second Regiment of Missouri Field Artillery, which was called into regular service when America entered the war. Truman served from August 1917 until May 1919. He was commissioned as a commander of a field artillery unit in France, where his unit fought in various campaigns including the Meuse-Argonne offensive in 1918, which was the biggest operation and victory for American forces in the war. Truman rose to the rank of colonel before the war's end.
After the war, Truman at first tried his hand at owning a men's clothing store, but this ultimately failed in 1922. Later in that year, Truman was appointed as a “judge” of Jackson County, Missouri, but the position was administrative rather than judicial and his role was similar to a county commissioner. From 1926 until 1934, Truman was the head judge of the county.
In 1935, Truman was elected as the Democratic senator representing Missouri, and he served throughout World War II. During his time as a senator, he led a committee that eventually bore his name: the Truman Committee. The committee's job was to look into military wastefulness and was able to save billions by exposing waste and fraud. Truman's leadership gained him national recognition that led to his being nominated as Roosevelt's vice president in 1945.