I very much recommend this article on Matthew Hoh, a Marine and Foreign Service Officer who recently resigned because:
“I have lost understanding of and confidence in the strategic purposes of the United States’ presence in Afghanistan,” he wrote Sept. 10 in a four-page letter to the department’s head of personnel. “I have doubts and reservations about our current strategy and planned future strategy, but my resignation is based not upon how we are pursuing this war, but why and to what end.”
Read more about him, as well as a copy of his resignation letter, here.
Update: Glenn Greenwald takes a closer look at the resignation letter.