Thursday in West Virginia, Barack Obama gave a speech laying out the economic costs of the Iraq War, which he estimated as up to $3 trillion (Linda Bilmes and Joseph Stiglitz’s estimate) and $10 billion a month. He listed the many things that money could have bought. Robert Menendez made similar points in the Democrats’ weekly radio address.

Americans disagree on whether to stay in Iraq and the best use of the money we’d save by leaving, but everyone should acknowledge that this is the way to argue about the war. The questions that consume the media, whether the surge worked, whether we’re making progress, and so on, are important, but they alone cannot determine whether we ought to continue the occupation. That depends primarily on cost-benefit analysis, however uncertain. (Moral questions matter too but are not meaningful when divorced from consideration of costs and benefits.)

Since the cost of staying is enormous, the backers of continuing American participation in the war should enumerate the benefits that justify it (along with the deaths and the shifting of our constitutional design towards unbalanced executive power). War boosters seem to understand the terrible burden of their position, as evidenced by their tendency toward wild, worst-case accounts of the consequences of American departure. In my view, the war wouldn’t be worth continuing even if the surge were working, which it isn’t.

But since we’re talking opportunity costs, what about the rest of the national security budget – you know, the other 80 percent of American security spending, now approaching three quarters of a trillion dollars, which is mostly spent to defend us against a couple weak conventional enemies? Like most other Democrats, Obama not only avoids complaining about regular defense spending, but backs the ongoing plan to expand the ground forces, which will add $15–20 billion in annual defense costs in the name of better executing future occupations like Iraq. I understand the political calculus here, but let’s not give the guy too many medals for political courage.

Democrats like Obama and Menendez also argue that Iraq is a reason that we are shortchanging state-building efforts in Afghanistan. This talking point illustrates the trouble with conventional foreign policy thinking on the so-called left. By saying that Afghanistan needs the medicine Iraq is getting, Democratic foreign policy leaders are rushing to repeat a mistake they rightly condemn. As Harvey Sapolsky, Chris Preble and I have argued, this thinking shows that the hubris that brought us into Iraq is essentially intact.

Defending American interests in Afghanistan requires nothing more than the absence of haven for international terrorists and an example made of those who offer it. The latter is a lesson well taught. Should it fail, a small ground force can target terrorist camps and supporters via raids and air strikes guided by intelligence, even if Taliban militias gain power in some regions. Those missions never required that Afghanistan become a modern nation, democratic, or even stable.

Instead of this realistic approach, the next President will probably expand a second no-end-in-sight war, one meant to assert the control of a statelet in Kabul over an unruly territory offering little historic basis for the word “nation.” Afghanistan is full of arms and grievances. It lacks the basics of statehood: a road network, a national energy grid, widespread patriotism, and tax collection. The notion that a 30 or 50 percent increase of Western forces and investment can transform Afghanistan into a peaceful, centralized state shows idealism of stunning tenacity. Obama talks more sensibility about these matters than John McCain, but he should apply some cost-benefit analysis to that spending too.