At age 18, Hisao "Koby" Kobayashi started his own produce-trucking business in conjunction with his family's melon farming business. Five days before Japan bombed Pearl Harbor, the U.S. Army drafted him into the Medical Corps. He was the second of four sons drafted by the U.S. military. His mother, sisters, and brothers went to Poston Detention Camp, and the Army detained his father separately in New Mexico.
At Camp Robinson, Arkansas, Kobayashi worked as a mail carrier and had the opportunity to visit Rowher and Jerome detention camps in his family car. His captain encouraged him to enter the Military Intelligence Service (MIS), and despite his misgivings in December 1943, he entered Camp Savage, Minn. Because of his limited Japanese and age (he was 25), he did not graduate until early 1945. From there, he shipped out to the Philippines:

> The Filipinos pointed at us and, yelling "JAPON! JAPON!" We had to shove them away. Due to the hostility of the natives, we were cautioned to stay away from the downtown area. [Autobiography]

One highlight of his work in the Philippines was the interrogation of a high-ranked Japanese officer.
Soon after Kobayashi's arrival in the Pacific War, the United States bombed Hiroshima and Nagasaki and Japan surrendered. He had served for four-and-a-half years and felt relief that the war finally was over. In September 1945, he arrived in Tokyo:

> I was shocked to see the destruction from the bombings. I saw people in a state of starvation. The women and children were hiding in the hills for they heard that the Americans were barbarians. However, it wasn't long before they started to return. The women were dressed in baggy pants and their stomach distended. They had been on a diet of sweet potatoes. [Autobiography]

In Tokyo, he searched out family members. His uncle, rich before the war, did not have heat in his home and bought rice on the black market. In November 1945, the U.S. military sent Kobayashi to Hokkaido.
After five months in Japan, Kobayashi worried that his family and their business would suffer in his absence, and he decided to return to the United States despite requests by the Army to stay. In February 1946, he returned to the mainland and soon married.