Dennis was killed in action in Vietnam. 

Here are some of the details as reported on the Virtual Vietnam Veterans Memorial site:

Sp-4 Dennis L. Magrie, son of Mrs. Joseph F. De Haven, of 6 N. Belfield ave., Havertown, who was killed in action in Vietnam on January 30, was awarded posthumously the U. S. Army Commendation Medal with the V device, it was announced Tuesday, May 8, 1968.
Specialist Magrie who was a grenadier with an infantry division near Dak To ran through heavy fire during an intense mortar and automatic weapon attack to aid wounded comrades and carry them to an aid station.
He then provided cover fire with his grenade launcher for American soldiers returning to their bunker until he was mortally wounded.
The commendation presented to Mrs. De Haven and her husband read:
"Specialist Four Magrie's personal bravery, professional integrity and exemplary devotion to duty are in keeping with the highest traditions of the military service and reflect great credit upon himself, his unit and the Unitied States Army."
Before his death, Magrie also had been awarded the Purple Heart, the National Defense Medal, Vietnam Service Medal and the Vietnam Campaign ribbon. - See more at:

http://www.vvmf.org/Wall-of-Faces/31953/DENNIS-L-MAGRIE