He served in the Vietnam War as a young lieutenant. He was once good friends with fellow Air Force Colonel Jack O'Neill. They fought side-by-side in Iraq during the Gulf War. During a mission in the Iraqi controlled city of Al Wafrah, Kuwait, O'Neill was injured by Iraqi soldiers, and believing he was killed, Cromwell put his team first priority, and left him behind. O'Neill would spend the following four months captured in an Iraqi prison. When he discovered O'Neill did survive, their friendship was over despite Cromwell's regret of leaving him behind.
As a result of being imprisoned, O'Neill soon developed a strong, overwhelming determination and a refusal not to leave his own people behind at any cost, a trait that Major/Lt. Colonel John Sheppard, the military commander would also develop during his time in Atlantis. (RPG: "Friends and Foes: Stargate Season Two") (SG1: "A Matter of Time")
He was assigned to lead the Pentagon Standby Strike Team, and was made aware of the Stargate's existence. He and his team were sent by The Pentagon after they lost contact with Stargate Command for several hours, which was in fact caused by an expanding time dilation field after the activate the Stargate to P3W-451, a planet facing destruction from a black hole. He thought the base faced a possible alien incursion, and stormed the SGC. After encountering O'Neill, he was made aware that it wasn't a hostile incursion. After the Pentagon ordered the base destroyed to possibly stop the increasing time dilation field, Cromwell and O'Neill volunteered to stay behind and activate the self-destruct. During that time, Cromwell asked O'Neill to forgive him, though O'Neill doesn't give it to him.
They are stopped from launch the self-destruct, as it wouldn't work. During the hours of thinking, Captain Samantha Carter came up with an idea to use a shaped charge to jump the wormhole to another planet. Cromwell and O'Neill repel to the Stargate and activate the bomb for the plan to work. However, on the way, a piece of glass was lodged on his rope, and after the iris gave way, and the rope snapped, he held on to O'Neill.
Knowing a single rope won't hold the two of them, Cromwell willingly lets go; the black hole's gravity pulls him through the Stargate and to P3W-451, presumably resulting in Cromwell dying. His sacrifice however, was not in vain, as the plan did work, and the shaped charge sent the wormhole to P2A-870. (SG1: "A Matter of Time")