If the individual is deceased, you can request a copy of their service records from the National Archives. If they are still living, they can make the request.
You probably won't find it. There was a fire years ago that burnt up many of the records of the World War 2 servicemen and women. Some of the wills were discarded when the servicemen were demobilized...
There are two good places. First depending on what information you have, you can contact the military personnel records in St Louis. MO. Second, you can contact the National Archives website and...
Do a Google search for World War 2 combatants. You'll find that almost every large nation was involved including the US, Germany, Japan, England, France, Russia (Soviet Union) Australia, and more