Well, at this point, even some of the generals are admitting that they blew it in planning this one:

The removal of the Iraqi government is likely to take longer than originally thought, Lt. Gen. William Wallace, the commander of the Army forces in the Persian Gulf, said today.

"The enemy we're fighting is a bit different than the one we war-gamed against, because of these paramilitary forces," General Wallace said. "We knew they were here, but we did not know how they would fight."

And Michael Gordon's anonymous sources at the Pentagon go further:

The Pentagon understood from the start that Mr. Hussein's forces would opt for an "urban-centric" defense. What the Pentagon did not understand was that the Iraqis planned to expand that strategy to include Nasiriya, Najaf, Samawa and other towns in southern Iraq.

The result was that after the American military raced north toward Baghdad, it discovered that it had a difficult and unexpected threat in its rear areas.

Of course, there were other generals, who had a rather different view, at least as early as last October, when Robert Novak wrote that

Hawkish civilians, in and out of the government, have been suggesting that Saddam Hussein's elite Republican Guard will throw up its arms in surrender. No serious person believes that. The question is whether an uprising of the persecuted Shia majority will be enough to overthrow the Baghdad regime without heavy application of U.S. force. If there is no effective revolt, the generals and their friends on Capitol Hill worry that the unknown plans may not call for sufficient U.S. forces.

So current circumstances were entirely foreseeable, and, in fact, foreseen. Rumsfeld just didn't want to hear about it. (As, in fact, are certain nasty future scenarios. Diana Moon thinks that a last-ditch streetfight through Baghdad is a "nightmare scenario"; in fact, it's the best reasonable case that I can see for the US, as Saddam's government has made it plain that they're not going down without that kind of a fight, and the population in Baghdad generally is far friendlier to his regime than the folks in, say, Basra. The question, at this point, is whether he somehow manages to drag us into something worse -- by dragging in some of his neighbors, for instance, or precipitating some kind of blowup with the Turks in the north).

This is bound to have some effect on the conservablogger cult of Dubya the deep strategist. But I'm sure they'll find an explanation. Even the Guardian is wondering... could the Dubya we've been looking at the last few months be one of his infamous body doubles?

Update: Michael O'Hanlon thinks the US has some tricks up its sleeve for making urban combat a bit less of a nightmare. Let's hope they work better than "shock and awe"... (via Matthew Yglesias)

And more: in the WaPo, the CIA says its analysts "thought there was a good chance we would be forced to fight our way through everything". The same article has a bizarre quote attributed to a "senior intelligence official", that the Fedayeen could take on more importance "if the media turns them into the equivalent of the black pajama Vietcong". As if, were the media not reporting that the Fedayeen were a hindrance, it would not be so...