Wendell Fertig (b.1901 - d.1975) was an American civil engineer in the Philippines, who became a leader of a guerilla force in the Japanese-occupied Southern Phillipine island of Mindanao during World War 2.

Fertig was commissioned as a Captain (reserve) in the United States Army Corps of Engineers at the outbreak of the Second World War in the Pacific (1941). Promoted twice by April 1942, Fertig - by then a Lieutenant Colonel - was sent from Bataan to Mindinao by General King (Luzon Force Commander) to assist General Sharp (Mindanao Force Commander). After the Philippines were surrendered to the Japanese in May 1942, Fertig decided to continue the fight.

Over the next two and a half years, Fertig created and ran United States Forces in the Philippines (USFIP) with escaped POWS, Filipino natives, rival guerilla groups, and soldiers who had refused to surrender. This was done despite Fertig having no formal military training (he was direct-commissioned into the military). Between 1942 and 1944, USFIP conducted numerous raids against the Japanese Occupation Forces on Mindanao in order to both sustain his operation with captured supplies, and to carry on a harassing operation against the Japanese. In order to be able to lead, Fertig had to promote himself to Brigadier General as he knew that no Filipino would follow a Lieutenant Colonel in a Guerilla operation. This self-promotion did not endear him to General Douglas MacArthur or his staff, but MacArthur did send logistical support to Fertig throughout 1943 and 1944.

From humble beginnings, USFIP became one of the best equipped and effective irregular units operating in World War 2. In fact, when the submarine USS Narwhal went to Mindanao in 1944 to deliver supplies, they were met by the uniformed band of USFIP playing "Stars and Stripes Forever".

Fertig was promoted to full Colonel by MacArthur and awarded the Distinguished Service Cross for his efforts. After the war ended, Fertig resumed his career as a civil engineer. He is widely regarded as a hero by the people of Mindanao, and was a highly respected figure among the U.S. Special Forces. He helped found the Army Special Warfare School at Fort Bragg, North Carolina.

A lot of military scholars are of the opinion that the failure of the military to promote Fertig to General Officer rank was a travesty perpetuated by General MacArthur's staff. By his efforts in establishing USFIP, Fertig ended up by 1944 commanding a force totaling 30,000 effectives (the equivilent of an Army Corps).

James Keats' book They Fought Alone accurately describes Fertig's guerilla operation.