"This...man you describe, He had no business surviving. A quarter of a million men and women dead on the battlefield..."
- – "Arthur" referring to David McIntyre
In 2245 he was assigned to the EAS Prometheus, during the first Earth expedition into Minbari space. When the expedition encountered a group of Minbari ships approaching with their gun ports open, Captain Jankowski panicked and ordered all forward batteries to open fire. As the ship's Gunnery Sergeant, David was the one to fire the shots that killed Dukhat and began the Earth-Minbari War. 
Although a court of inquiry on the incident cleared him of all charges, he suffered bouts of depression because he felt responsible for starting the war. Despite that, he continued to serve bravely throughout the war, earning the Silver Star for Valor before volunteering for the Battle of the Line and emerging as one of only two hundred or so survivors. McIntyre was honorably discharged from Earthforce on February 9, 2253. For years afterwards he continued to be plagued by depression and anguish over his self-perceived responsibility for the entire Earth-Minbari conflict and in 2260, on the fifteenth anniversary of the first contact with the Minbari, he travelled to Babylon 5, apparently under the delusion that he was King Arthur, dressed in armor and carrying a sword. While on the station, he helped an old woman regain a valuable picture frame. In the fight to get it back, he befriended G'Kar whom he knighted while they shared drinks. Stephen Franklin helped David deal with his denial by confronting him with the truth about his identity. Further, he asked Delenn to take his sword from him, symbolically relieving him of his burden of guilt and pain by forgiving his actions. This was also symbolic of how the fictional King Aurthur returned the sword to the Lady of Lake before going to Avalon. Shortly afterwards, with the help of Doctor Franklin, he was treated and returned to full mental health. Shortly afterwards, he decided to work with the Narn Resistance in an unspecified leadership role. Citizen G'Kar agreed to David's request and made the necessary arrangements for him to travel to the Narn Homeworld to help organise and motivate the resistance.