“To come all that way, rifle in hand, with two thousand people marching at my heels, and then to trail feebly away, having done nothing—no, that was impossible.”

What purpose does the end phrase separated by the em dash serve in this sentence?

A. The phrase is meant to draw the reader away from the sentence.
B. The phrase is meant to qualify the author’s actions in relation to his first statement.
C. The phrase is meant to simplify and clarify the author’s first thought.
D. The phrase is meant to contrast what is possible and what is impossible.