From the Wiki:
America's entry into World War II resulted in a deluge of support for the war effort from all sectors of society, and Hollywood was no exception. Wayne was exempted from service due to his age (34 at the time of Pearl Harbor) and family status (classified as 3-A – family deferment). Wayne repeatedly wrote to John Ford saying he wanted to enlist, on one occasion inquiring whether he could get into Ford's military unit.[39] Wayne did not attempt to prevent his reclassification as 1-A (draft eligible), but Republic Studios was emphatically resistant to losing him since he was their only A-list actor under contract. Herbert J. Yates, President of Republic, threatened Wayne with a lawsuit if he walked away from his contract,[40] and Republic Pictures intervened in the Selective Service process, requesting Wayne's further deferment.[41]
U.S. National Archives records indicate that Wayne, in fact, did make an application[42] to serve in the Office of Strategic Services (OSS), precursor to the modern CIA, and had been accepted within the U.S. Army's allotted billet to the OSS. William J. Donovan, OSS Commander, wrote Wayne a letter informing him of his acceptance into the Field Photographic Unit, but the letter went to his estranged wife Josephine's home. She never told him about it. Wayne toured U.S. bases and hospitals in the South Pacific for three months in 1943 and 1944,[43] with the USO.[44][45][46] During this trip, he carried out a request from Donovan to assess whether General Douglas MacArthur, commander of the South West Pacific Area, or his staff were hindering the work of the OSS.[21]: 88 Donovan later issued Wayne an OSS Certificate of Service to memorialize Wayne's contribution to the OSS mission.[21]: 88 [47]
By many accounts, his failure to serve in the military later became the most painful part of his life.[39] His widow later suggested that his patriotism in later decades sprang from guilt, writing: "He would become a 'superpatriot' for the rest of his life trying to atone for staying home."[48]