On 10 March 1967, Captain Dethlefsen flew on a mission against the steel works at Thai Nguyen some fifty miles north of Hanoi, North Vietnam, with three other F-105 Thunderchief pilots. Their task was to go in ahead of a strike force of fighter-bombers and attack the surface-to-air missile (SAM) complex, antiaircraft guns, and automatic weapons ringing the target. On the first pass against these defenses the F-105 flight leader was shot down and his wing man was forced to withdraw with severe battle damage. Captain Dethlefsen decided to continue the attack on his own. As he maneuvered, he evaded an intercepting MIG-21 by flying into heavy enemy antiaircraft fire but his F-105 was seriously damaged.

Captain Dethlefsen nonetheless made repeated strikes with his wing man against the defense positions, even after they became obscured by the smoke and dust of the exploding bombs being dropped by the fighter-bombers. The bombing completed, the strike force withdrew but the two F-105's remained over the target. Evading a second MIG, Captain Dethlefsen was diving through the obscuring haze to locate the missile complex when he was again hit by flak. Making a final dive bombing attack and a strafing run with 20mm cannon fire, Captain Dethlefsen effectively destroyed two missile sites before leaving for home in his battered F-105.

For this action, Captain Dethlefsen was awarded the Medal of Honor. The presentation was made at the White House by President Lyndon B. Johnson on 1 February 1968.