He flew with the 560th Tactical Fighter Squadron at McConnell Air Force Base from February 1965 to October 1966, when he deployed to Southeast Asia.

On March 10, 1967, Dethlefsen along with his EWO Kevin Gilroy, flying the number three aircraft, and three other F-105s of Lincoln flight, flew ahead of a strike force of 72 fighter-bombers (F-105 Thunderchiefs from Korat and Takhli, and F-4 Phantoms from Ubon) heading to the Thai Nguyen iron and steel works.

Their job was to attack the surface-to-air missile complex, antiaircraft guns and a ring of automatic weapons guarding the target.

On the first pass, his flight leader (F-105F 63–8335, piloted by David Everson and Jose Luna, both POW) was shot down by 85mm AAA fire and his wingman was forced to withdraw with severe damage.

[4] Dethlefsen could have pulled out of the mission on a number of occasions: when attacked by MiGs, when he and his wingman were hit by flak, or when the smoke of battle made it difficult to locate the enemy.

The defensive network was situated to dominate the approach and provide protection to an important North Vietnamese industrial center that was scheduled to be attacked by fighter bombers immediately after the strike by Maj. Dethlefsen's flight.

His action in rendering ineffective the defensive SAM and antiaircraft artillery sites enabled the ensuing fighter bombers to strike successfully the important industrial target without loss or damage to their aircraft, thereby appreciably reducing the enemy's ability to provide essential war material.