How does the opening of Dances with Wolves depict Dunbar (& does it reveal whether he can be trusted with assumptions about Native Americans)?

The opening of Dances with Wolves is
very bloody indeed:  bloody and full of irony.

John Dunbar
is wounded in battle during the Civil War.  It is immediately obvious that he needs his
leg amputated, but after seeing a soldier with only one leg on crutches (and, perhaps,
hearing that there was no ether to dull the pain), Dunbar decides to ram a boot onto his
bloody foot and end his own life.  To do this, he takes a horse and rides directly in
front of the southern army.  He makes this ride twice, but he survives.  The situational
irony is, his sacrifice creates a distraction for the North to win that particular
battle and Dunbar is awarded a medal for his bravery.  The dramatic irony is that we as
the audience know that Dunbar wasn't trying to be brave.  He was, in fact, a
coward.

In once sense, the movie is Dunbar's transformation
from cowardliness to bravery.  It takes his experience with the frontier and, of course,
with the Sioux to teach him this.  To answer your question, this is only the beginning
of the story, so Dunbar is seen as a proud man (too proud to lose his leg) and
determined (determined to lose his life instead).  Although he is mistaken for a man of
honor, he is not.  However, it does make his superiors think he can be trusted with
information.

The other irony about these opening scenes is
that they reveal absolutely nothing about whether John Dunbar can truly be trusted with
assumptions about Native Americans.  Dances with Wolves is not
really a movie about the Civil War at all.  Instead, it is a movie about John Dunbar's
journey from the "American way" back to the more honorable "Native American way."  This
is how John Dunbar both finds and achieves true bravery.