He graduated from Navigator School in June 1941 and on the same month, Nielsen was assigned to the 17th Bomb Group at McChord Field in Washington, which was equipped with the North American B-25 Mitchell bomber.

[4][3][5] After the Japanese attack on Pearl Harbor on December 7, 1941, the 17th BG immediately began anti-submarine patrols off the coast of Oregon and Washington.

Running low on fuel due to the early launch of the raid, the B-25s failed to reach any of the designated safety zones in China.

Second Lieutenant William J. Dieter (bombardier) and Sergeant Donald E. Fitzmaurice (gunner) drowned when the aircraft ditched into the sea, while Nielsen, Hallmark and co-pilot Robert J. Meder managed to swim ashore.

[8][5] On April 27, as they tried to reach safety with the help of friendly local Chinese, all three men were captured by Japanese troops and interred as POWs in Shanghai, along with crew of the sixteenth bomber.

On August 20, 1945, Nielsen and other prisoners were rescued at the end of the war by an Office of Strategic Services para-rescue team and brought back to the U.S.

[12] During his decade with SAC, Colonel Nielsen helped the command develop key operational innovations, including radar navigation bombardment, air refueling employing the flying boom, and electronic countermeasures.

He helped integrate "fail safe" and other emergency war order procedures into SAC's unique set of flight profiles.