There's a couple of great podcasts about this on the "Connecting with Walt" DIS Unplugged podcast series. Walt's mother refused to give permission for him to join the Armed Forces in WWI, so Walt forged the birth certificate and changed it from 1901 to 1900 so he could join the Red Cross. Then just before his Red Cross unit was set to be shipped overseas, he got that horrible flu that killed millions of people, so his departure was delayed.
1 of the "Connecting with Walt" podcasts talks about how the Disney studios participated in making war films during WWII. It's really interesting.