I am convinced that our efforts have had a positive impact on our security.
The question is not whether the impact is positive or negative. The question is
how much of a positive impact is it having. It's a question of a
cost-benefit analysis.
Look: I have no doubt that wearing a ballistic vest and some plates to work will reduce the chance of me being shot dead. But the discomfort relating to wearing a vest and its cost, related to the chance of someone shooting at me (less than 1%), makes me reconsider this fashion choice. Does it mean I do not value my own life? Of course it doesn't.
The United States used to go around the world, protecting its allies, kicking butt, and installing liberty wherever it went. It might have been not ideologically perfect, but today, Germany, Japan, Italy, the Ukraine, the Baltic States, and countless other countries are relatively free because of what America did - because America fought in WW2 and waged the Cold War.
But now you're suggesting that America should continue investing money, blood, freedom, political capital and international prestige for as many as eight more year - effectively doubling the current expense - in the Afghani mountains, and what we'll have for it is... creating a new Pakistan? For two trillion dollars?
I can think of a dozen things you can buy for two trillion dollars that are shinier than a second Pakistan. One is more than enough.
And this is the best-case scenario. What will likely happen that - five, six years from now - the American public will get tired and America will leave. And then all the money, lives, effort spent will be in vain. Worse, the loss - and its inevitable psychological impact - will be used, like Vietnam, to discourage the American nation, to sap its spirit, to prevent it from other, worthier exploits.