Dances With Wolves:
Dunbar's journey to the frontier is facilitated by his commanding officer being a laudanum or morphine addict.
It's not apparent at first, and I've had to rewatch it many times, but the man who signs his papers and later kills himself has lots of little bottles all over his desk, if you notice carefully while he's posting Dunbar to Fort Sedgwik. He's an opiate addict, common at the time, quite possibly sent out west because he became one during the war, incompetent AND incontinent, and was given a useless desk job that he still mishandled. In some ways he's like Dunbar. Perhaps he too wanted to see the frontier and escape the horrors of the war.
so he probbly knew Sedgwick's officers weren't reporting back, and that he was sending Timmons and Dunbar on a suicidal fool's errand, but he did it anyway, and then killed himself without telling anyone where Dunbar was heading. When Timmons died, Dunbar was completely isolated from his former world and no one knew what happened to him.
I always got part of that, just hadn't realized the whole laudanum thing.