My grandfather was a World War II veteran. He was stationed in New Guinea and the Philippines. He always told me that he kept a journal during his time overseas but before he got to come home, his unit had to destroy anything that they had written about their time there. This always bothered me and it bothers me even more now that my grandfather has passed away. I can't seem to find any other situations where this happened and I haven't heard this from anyone else. Is it possible that he destroyed it on his own accord because he wanted to leave the war there, so to speak? Are there any recorded instances of WWII veterans being forced to destroy their journals? Please help point me in the right direction if you have any reading that I can do on this subject.
The keeping of a journal or diary was prohibited in theater for American forces. The timeline of the destruction of your Grandfather's journal seems off, but I'm not in a position to argue. The reason being is that the journals and diaries were a security risk to have in the event of capture.
For example Eugene Sledge, the author of With the Old Breed and introduce to mainstream audiences as a main character in HBO's the Pacific, had to hide his journal in his copy of the New Testament. He wrote his notes hidden in the margins and was able to use them post-war to reconstruct events to write his memoir. In the memoir he mentions the ban on diaries and journals, and his method to skate around regulations.
Edit: If you'd like more information on your Grandfather's service I'd recommend getting his service records from the National Archives. At the very least you should be able to get information on where he trained, what his military specialty was, what unit he was with overseas, and awards. Once you have his unit and awards, I can help research the unit and figure out what he was likely involved with while in the Pacific.