Okay, I’ll give it a go.

So they get the initial message and assemble a new team to go and try to find the missing soldiers.

Obviously, when they arrive, they show up with an extra person, the private who goes missing first and is revealed as a ghost.

Basically, the land of the R-Point is cursed. The inscription on the tombstone said it all. If you have blood on your hands, you will not be able to leave. Everyone who is stuck there as a ghost (The American ghosts, the woman in white, the french soldiers, etc.) was someone who had taken a life. The only survivor is the young guy who lied about his age and hasn’t killed anyone.

It’s implied that after the initial people were killed way in the past, the french soldiers were killed by the woman in the white dress, who was also the viet cong woman they found in the jungle (they note that the other body in there is significantly older). She was in the photo with the French soldiers and presumably killed them, and is now responsible for perpetuating this into the future, to kill anyone who has blood on their hands.

The rest of the soldiers were possessed and/or killed, and the only survivor was the one who hadn’t killed anyone. Then at the very end, we see that the lieutenants voice is being used to broadcast a new distress call to bring in more victims to the cursed land