He was inducted into the Philippine Constabulary (PC) where he was captain when World War II broke out in December 1941. The PC was then integrated to the United States Army Forces in the Far East (USAFFE). He was assigned with the 71st Infantry based in Northern Luzon.
The USAFFE was surprised when the Japanese army landed in Aparri, Cagayan. Because of this, Nakar's 71st Infantry could not join the other USAFFE units in Bataan where they were supposed to wait for the US reinforcements. To escape the Japanese forces, they left Bauang Bay, La Union for Aritao, Nueva Vizcaya where they combined forces with the 26th Cavalry and the 11th Infantry. He formed a guerilla force and became its lieutenant colonel.
In January 1942, they attacked Tuguegarao, Cagayan, killing 100 Japanese soldiers and destroying three enemy planes on the ground. Because of their success, they were then redesignated as the 14th Infantry. They attacked Japanese garrisons and ambushed Japanese patrols.
When Corregidor and Bataan fell in May 1942, Nakar refused to surrender. His group moved to Madela, Nueva Vizcaya, where he organized an effective intelligence system, covering Isabela, Nueva Vizcaya, and Pangasinan.
However, in November 1942, Nakar was betrayed by a traitor, leading to his capture and imprisonment. He refused to sign his surrender papers and to swear allegiance to the Japanese. When the Japanese officials allowed him to speak in public as part of their propaganda campaign, he condemned them for their abuses and cruelty. He was tortured and imprisoned at Fort Santiago and executed on 29 September 1942.
After his death, Nakar was awarded the Distinguished Conduct Star in July 1945. A military camp in Lucena, Quezon (now the headquarters of the Southern Luzon Command of the Armed Forces of the Philippines) was also named in his honor.
- Agoncillo, Teodoro A. 1965. The Fateful Years, Japan's Adventures in the Philippines Volume II. Quezon City: RP Garcia Publishing Company.
- Baclagon, Uldarico S. 1965. They Served with Honor. Quezon City: DM Press Inc.
- National Historical Commission of the Philippines. 2015. Guillermo Nakar (1905-1942). Accessed 24 April 2021. https://nhcp.gov.ph/resource/filipinos-in-history/martyrs/.
- Reyes, Pedrito and Jose Domingo Karasig. 1954. Pictorial History of the Philippines. Manila.