This passage from his letter is to me the heart of the argument for curtailing rather than expanding America's stake and commitment in Afghanistan:
"I find specious the reasons we ask for bloodshed and and sacrifice from our young men and women in Afghanistan. If honest, our stated strategy of securing Afghanistan to prevent al-Qaeda resurgence or regrouping would require us to additionally invade and occupy western Pakistan, Somalia, Sudan, Yemen, etc. [My emphasis.] Our presence in Afghanistan has only increased destabilization and insurgency in Pakistan where we rightly fear a toppled or weakened Pakistani government may lose control of its nuclear weapons. However, again, to follow the logic of our stated goals we should garrison Pakistan, not Afghanistan. More so, the September 11th attacks, as well as the Madrid and London bombings, were primarily planned and organized in Western Europe; a point that highlights that the threat is not one tied to traditional geographic or political boundaries."
The United States entered Afghanistan -- properly and with every moral and practical justification -- to disrupt, punish, and kill groups that had planned the 9/11 attacks. It is now in a mess in Afghanistan largely because of the crucial misjudgment nearly eight years ago to shift effort and attention to Saddam Hussein and Iraq. Not everything in foreign affairs can be explained by logic. But as Hoh argues, if we're serious in thinking we can now eliminate terrorist threats with our troops in Afghanistan, then logically we must also send them to Pakistan and beyond. And if we're not serious, then how can we keep them there?
For-the-record point: resignations on principle are vanishingly rare in U.S. government practice. It's much easier to keep your head down, protect your career prospects, and when it's over say that you had been against [failed policy xx or yy] all along. Apart from the merits of his argument, Hoh deserves respect for taking this step so forthrightly. Each person who does so creates an example for others to reflect upon.