Conservatives’ increasingly desperate defenses of the Iraq mess now turn more and more to a simple idea, “Well, it’s too late to stop now.”

They seem to have forgotten the notion of sunk costs, which in economics is the term for any costs already invested into a project which have to be conceded if the project is deemed a failure.

For example, no corporation in America would say, after spending millions to launch a new product, “Well, it’s failing miserably, but we’ve already spent so much money developing and marketing the damn thing, let’s just keep letting the losses pile up.”

No, after a certain point, you stop the bleeding. Call it a valiant try. Call it a mistake. Cut your losses and move on, to hell with your “credibility.”

This is especially the case with an optional war. In a sense, Dubya turned an optional war into a must-win war, because of the seeming alliance between Iraqi insurgents and foreign Islamic fundamentalits. How much of that is actually happening, though, is very unclear.

Still, the weapons threat — assuming there was one — is most assuredly gone now. While attacks targeting civilians in Iraq are obviously terrible, they are not obviously a priority for American foreign policy. Turn over authority ASAP to the Iraqis and come to terms with the new government on keeping a strike force in the country to hunt down any terrorist groups.

It’s this type of smaller-scale, more targeted action that the war on terror seemed to be headed in originally, before Dubya & Co. became obsessed with remaking the entire Middle East. There’s not much of a guarantee that this would go well, but it would mean bringing home the vast majority of the troops, getting out of the nation-building and recognizing when your costs are sunk.

Unlike in the private sector, however, there’s much less incentive for politicians to admit their mistakes, much less work to lessen the damage caused by them. It must be daunting for them to be faced with the fact that nine out of 10 things they’ll try will fail.

The only lever, ultimately, is the ballot box, as imperfect an institution as it might be. Yet the leading presidential candidate for the other major party also wants to stick it out in Iraq, albeit under NATO command. I’m sure it won’t be any problem to get the Europeans on board now that Iraq is a flaming disaster.