We love our principles. They are by nature simple and clear-cut. They allow of no diversion, no excuse. The world, by contrast, is infinitely complex, as are the human beings who inhabit it. I wish it were otherwise, but clear moral choices are muddied when it comes to the practical reality of events on the world stage, where the lives of millions of human beings are at stake. In such situations, those we have chosen to lead us are left with agonizing choices whose resolution may subvert our most cherished principles. None of which excuses us--or them, of course--from the necessity of striving in every way we can to hold ourselves to those principles in which we deeply believe.
It's an ongoing process. We make some good choices, some bad. It's often hard to tell which is which, to judge from the results. But, as I think Jean-Paul Sartre once said--correct me if I'm wrong: we are engaged. We cannot but stumble forward in the struggle, doing the best we can. This is why, despite all my personal misgivings--indeed, my personal revulsion--I cannot fault the president for failing to order all our troops, immediately, out of Afghanistan.