He was promoted to major by war's end, age 28, and was awarded the Distinguished Service Cross by General Douglas MacArthur.

Lapham was the third person, after President Franklin Delano Roosevelt and MacArthur, to receive the Philippine Legion of Honor.

[2]: 140 A graduate of the University of Iowa in 1939 with an ROTC 2nd lieutenant's commission in the Army Reserve, Lapham worked for the Chicago branch of the Burroughs Corporation before signing up for active duty in May 1941.

Lapham joined Major Claude A. Thorp in organizing "a raiding party that would slip through Japanese lines", with the objective of sabotaging Clark Field, and gathering intelligence for General MacArthur.

On January 27, 1942, along with Thorp and a dozen others, Lapham slipped through Japanese lines and headed north into the Zambales Mountains.

They attacked a Japanese convoy near Olongapo, possibly killing several soldiers, but failing to capture food and supplies.

[1]: 9–22 Lapham and Sergeants Albert Short and Esteban Lumyeb, decided they would journey northward as they had heard rumors that Filipino resistance to the Japanese occupation was developing there.

Lapham left Short and Estipana there and went on with Lumyeb ten miles north to establish another camp in Umingan, Pangasinan.

In exchange for local support, Lapham promised that, to avoid Japanese reprisals, he would not fight near his bases and that he would control the bands of outlaws and former soldiers who were ravaging the area.

Anderson and Volckmann established their bases in mountain redoubts, difficult to access and relatively safe from Japanese assault.

He was clean shaven, attempted to dress neatly in semi-military clothing, and avoided liaisons with Filipina women.

[3]: 98–99, 163–164, 191 [1]: 14, 50, 70 In 1943 and 1944, the estimated 13,000 Filipinos under Lapham's command in the Luzon Guerrilla Armed Forces (LGAF), engaged in "harassing the Japanese more than they had in 1942.

[1]: 184 On January 26, 1945, Lapham made an emergency visit to U.S. forces near Lingayen Gulf, warning that the 513 American prisoners of war (POWs) in the Cabanatuan camp might be executed by the Japanese.

Pajota ambushed a relief mission of several hundred Japanese soldiers which was instrumental in ensuring a successful rescue.

[1]: 243 In 1947, Lapham returned to the Philippines for five months as a consultant to the U.S. on the subject of compensation to Filipinos who had served as guerrillas during the war.

He said the U.S. Congress was "niggardly" with the Philippines, providing less money for rebuilding than that spent in many other countries, putting conditions on Philippine independence that favored U.S. business and military interests, and backing corrupt Filipino politicians who protected American, rather than Filipino, interests.