I watched Flags first..
The Premise on Flags is the back story of the men that raised the flag on Suribachi. The Premise on 'Letters' is the Japanese perspective on their realization and national determination that they would not be leaving that island alive.
... bear in mind the horror of what you'll see is just the tip of the Pacific Crisis iceberg. It wasn't till Okinawa that the 'handwriting' on what an Invasion of the Japanese mainland would entail. Iwo was rough.. Okinawa was far, far worse.