What a difference a day makes - especially in war.

Less than a week ago, U.S. troops seemed to be stalled 50 miles outside Baghdad. Coming on the heels of the lightning-fast opening U.S. strike, this pause prompted a round of public hand-wringing: Would the war drag on longer than expected? Had military planners overplayed their hand? The dreaded "Q" word - quagmire - began to surface.

But now, with our troops again moving like quicksilver to the gates of Baghdad, the mood has switched again. The tremendous success of the last 24 to 48 hours leaves a euphoric sense that the end may be near for the Butcher of Baghdad.

And indeed it might. Given the exemplary discipline, training and professionalism of our troops, we have every reason to be optimistic.

But as Defense Secretary Donald Rumsfeld said yesterday, there likely will be "difficult days ahead" for coalition forces as the assault on Baghdad begins. We must brace ourselves for this reality.

Of course, there is much to be pleased about. Coalition forces control at least 45 percent of Iraq and more than 95 percent of its airspace. They have driven 300 miles and reduced the Iraqi army to a mere shell of its pre-conflict self.

They have protected oil wells, secured their rear area and sharply limited both civilian and coalition casualties. Some 100,000 troops are on their way to Iraq as reinforcements.

But Saddam Hussein is a dangerous and desperate man - a thug and a murderer who, as Rumsfeld said yesterday, has probably "killed more Muslims than any person on the face of the earth." He has used weapons of mass destruction on his own people and his neighbors in Iran. His henchmen are torturers and rapists.

There is little Saddam won't stoop to - and now his back is against the wall.

The danger of chemical or biological weapons being unleashed on our troops is ever-present. Saddam may also step up the guerilla-warfare tactics we've seen so far, such as fake surrenders and small, hit-and-run attacks on supply lines.

Or his troops may escalate their atrocities against the Iraqi people or try to drag coalition forces into bloody urban warfare in Baghdad. We know they have thousands of Russian-made Kornet anti-tank missiles - a lightweight weapon capable of inflicting serious damage on the M-1 Abrams tanks our troops are using.

Saddam's capacity for brutality and perfidy is limitless, and he may be delusional in believing he can win: Not militarily, but politically, by dragging out the conflict, building Islamic rage and finding proxies to help him cut a deal with the coalition.

But Bush administration officials have made it clear that Saddam's surrender must be unconditional. (There will be "no deal, not a chance," Rumsfeld said yesterday.) The regime must go, and its weapons of mass destruction must be destroyed.

We should expect Saddam to make his last stand in Baghdad surrounded by his Special Republican Guard, clouds of chemical agents and street-to-street fighting in an effort to push the coalition death-count as high as possible. We may experience human shields, suicide bombers, booby traps and snipers.

Iraqi troops may even don British and U.S. uniforms to commit atrocities against the citizens of Baghdad, especially the Shia, hoping that the world will believe that these acts were perpetrated by coalition forces.

In short, Saddam is going down, but may well try to take down as many of us as he can with him. We must be prepared for this possibility on the battlefront as well as on the homefront.

Which isn't to say that our troops won't win - it's really just a matter of time - or that Saddam is incapable of miscalculating. He allegedly has watched the movie "Black Hawk Down" a number of times and believes we will make the same mistakes we made in Mogadishu in 1993 this time around as well.

He is wrong. We've studied Mogadishu, Northern Ireland and the Arab-Israeli conflicts, and we're ready.

War is unpredictable, and the conflict may be over at any moment. The fissures in the regime's façade are deeper and more noticeable everyday. We are well on the road to disarming Iraq and ending this brutal dictatorship.

Yet that unpredictability cuts both ways. Despite our success, we must temper our expectations and be sober about the potential challenges that lie ahead on the road to victory.

-Peter Brookes, a former deputy assistant secretary of defense, is a senior fellow for national security affairs at The Heritage Foundation (www.heritage.org).