Why did private Ryan have a flash back of Omaha beach if he wasn't on it?

I assume you mean the beginning of the movie when the elderly Ryan begins to think back to the landing. This was somewhat of a trick. As you recall, the movie ends with a close-up of Private Ryan morphing into the elderly Private Ryan.

Well, it was a little mis-leading when the movie started and showed a close-up of Ryan and then switching over to the Captain. I knew it wasn't the same person as I was aware that Private Ryan was in the 101st Airborne Division---but the Captain it showed was a Ranger in a landing craft. I see this more as the elderly Ryan was thinking back to what that day must have been like that cost so many casualties---as he stood there among all the graves.

He was not re-living it from his experience.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.