Citation... For conspicuous gallantry and intrepidity above and beyond the call of duty. In the attack on the approaches to the city of Cassino on 3 February 1944, 2d Lt. Riordan led 1 of the assault platoons. Attacking Hill 175, his command was pinned down by enemy machinegun fire from the hill and from a pillbox about 45 yards to the right of the hill. In the face of intense fire, 2d Lt. Riordan moved out in full view of the enemy gunners to reach a position from where he could throw a handgrenade into the pillbox. Then, getting to his knees, he hurled the grenade approximately 45 yards, scoring a direct hit.

The grenade killed 1 and wounded the other 2 Germans in the nest and silenced the gun. Another soldier then cleaned out the enemy pillboxes on the hill itself, and the company took its objective. Continuing the assault into Cassino itself on 8 February 1944, 2d Lt. Riordan and his platoon were given the mission of taking the city jail house, one of the enemy's several strongpoints. Again 2d Lt. Riordan took the lead and managed to get through the ring of enemy fire covering the approaches and reached the building. His platoon, however, could not get through the intense fire and was cut off. 2d Lt. Riordan, aware that his men were unable to follow, determined to carry on single-handed, but the numerically superior enemy force was too much for him to overcome, and he was killed by enemy small-arms fire after disposing of at least 2 of the defenders. 2d Lt. Riordan's bravery and extraordinary heroism in the face of almost certain death were an inspiration to his men and exemplify the highest traditions of the U.S. Armed Forces.