When the Japanese attacked Pearl Harbor on December 7, 1941, and the United States officially entered World War II, he enlisted.

[1] On his first combat mission, he photographed the fighting in North Africa, followed by the invasion of Sicily and then on to the Italian mainland.

[1] He parachuted "into France and Germany ... to take pictures of bridges, roads, rivers, railroads and even a V-1 launch site".

As dawn came up, he witnessed, and filmed, the D-Day invasion of Omaha Beach by the United States Army - the only known Allied footage from the German perspective.

Hjorth also shot pictures of Buchenwald concentration camp and the aftermath of an Axis atrocity in France (the corpses of dozens of civilians burned alive).