A WWII veterans discharge document is the best source for learning the facts of his time in service. When starting your research on a WWII veteran, the discharge document is the first thing you should obtain. See "Locating & Obtaining a copy of a WWII Veterans Discharge Document" in another post in this thread section.
One important point i must make concerns an old adage: ' Things May Not Be What They Seem'
The unit shown in Box 6 is a common source of confusion for some people starting their research. The unit shown in Box 6 may NOT be the unit the veteran served in during his or her entire time of service, but may only be the LAST unit he/ she was assigned to and discharged from. For example, many men in the Army Air Corp who flew on bomber crews have an Army Air Forces Base unit ( xxx AAF Base Unit) listed in Box 6. Thousands of high point infantrymen in Europe whose divisions were being redployed to the Pacific or staying for Occupation Duty were transferred to other units to be shipped home and this 2nd unit will be the one shown in Box 6.
This help document is a little lengthy so i put it in a .pdf