Answer by taking a quote from the following article:

De Benneville "Bert" Bell (February 25, 1895 - October 11, 1959) was the National Football League (NFL) commissioner from 1945 until his death in 1959. As commissioner, he introduced competitive parity into the NFL to improve the league's commercial viability and promote its popularity, and he helped make the NFL the most financially sound sports enterprise and preeminent sports attraction in the United States (US). He was posthumously inducted into the charter class of the Pro Football Hall of Fame. Bell played football at the University of Pennsylvania, where as quarterback, he led his team to an appearance in the 1917 Rose Bowl.

Bell entered Penn in the fall of 1914 as an English major and joined Phi Kappa Sigma. In a rare occurrence for a sophomore, he became the starting quarterback for Penn's coach George H. Brooke. On the team, he also was as a defender, punter, and punt returner. After the team's 3-0 start, Bell temporarily shared possession of his quarterbacking duties until he subsequently reclaimed them later in the season, as Penn finished with a record of 3-5-2.  Prior to Penn's 1916 season, his mother died while he was en route to her bedside. Nevertheless, he started the first game for the Quakers under new coach Bob Folwell, but mixed results left him platooned for the rest of the season. Penn finished with a record of 7-2-1. However, the Quakers secured an invitation to the 1917 Rose Bowl against the Oregon Ducks. Although the best offensive gain for Penn during their 20-14 loss to Oregon was a 20-yard run by Bell, he was replaced late in the game at quarterback after throwing an interception.  In the 1917 season, Bell led Penn to a 9-2 record. Afterwards, he registered with a Mobile Hospital Unit of the US Army for World War I and was deployed to France in May 1918. As a result of his unit participating in hazardous duty, it received a congratulatory letter for bravery from General John J. Pershing, and Bell was promoted to first sergeant. After the war, Bell returned to the United States in March 1919. He returned to Penn as captain of the team in the fall and again performed erratically. The Quakers finished 1919 with a 6-2-1 record. Academically, his aversion to attending classes forced him to withdraw from Penn without a degree in early 1920. His collegiate days ended with his having been a borderline All-American, but this period of his life had proven that he "possessed the qualities of a leader."

What did he do after the war?
Bell returned to the United States in March 1919. He returned to Penn as captain of the team in the fall and again performed erratically.