Private First Class Lassan was a member of Company L, 3rd Battalion, 8th Cavalry Regiment, 1st Cavalry Division. He was listed as Missing in Action while fighting the enemy south of Sunchon and north of Pyongyang, North Korea on 2 December 1950. He was presumed dead on 31 December 1953. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Lassan was awarded the Purple Heart, the Combat Infantryman Badge, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.