The incident occured just before 4:00 a.m. Sunday, when three sailors approached an apparent homeless man who was seated in the McDonald's located at 3620 N. Clark Street and they began "having words," according to a Belmont District police lieutenant.
The brawl escalated and the man stabbed a 20-year-old sailor with a steak knife, the lieutenant said.
Officials are trying to determine if the man was the aggressor or if he was acting in self-defense. The 20-year-old sailor was taken to Advocate Illinois Masonic Medical Center in an unknown condition.
Authorities said they would check video surveillance and talking to several witnesses of the incident.
The man who police have in custody had not been charged as of 11:15 a.m. Sunday. Belmont Area Detectives are investigating.