WASHINGTON -- U.S.-led invading forces are closing in on the hardest part of the Iraq war.
This is when they come up against Saddam Hussein's best troops, his Republican Guard.
This is the time, as the Iraqi president and his associates feel the noose tighten, that some say Saddam would be most likely to lash out with chemical weapons.
This is when foreign forces could be drawn into potentially the bloodiest combat: urban combat on the streets of Baghdad.
But first comes the crucial phase of dealing with Baghdad.
"It looks like it's going to be messy," said military analyst Francois Boo of GlobalSecurity.org.
Preparing the way for the 3rd Infantry Division advancing rapidly northward, coalition aircraft stepped up air strikes Monday against Republican Guard positions at the capital's southernmost outskirts.
Administration sources had said that as of Saturday, Republican Guard formations were holding around the capital, with no reported surrenders or defections. That's where three Guard divisions are posted, believed to number as many as 80,000 troops.
Strikes during the weekend and on Monday destroyed equipment and inflicted casualties among the defenders, Pentagon officials said, but they would give no details.
Still, troops should be prepared to fight for Baghdad, said Loren Thompson, analyst with the Lexington Institute, especially considering how the U.S. and British advance has gone so far. Thompson noted that war planners had hoped that months of flooding wide areas of Iraq with millions of leaflets would prompt massive surrenders among Iraqi troops and encourage a popular uprising among civilians. Neither event occurred, and some Iraqi forces -- even relatively ragtag outfits in the countryside -- have fought tenaciously.
Even so, Thompson said he expects the divisions posted on Baghdad's southern approaches to be substantially defeated by Wednesday or Thursday.
"The 3rd I.D. will either drive right into Baghdad or it will pause on the outskirts, depending on what intelligence shows about the resistance," Thompson said of the U.S. infantry division. "If they are facing signs of resistance, the city is going to take a bigger pounding from the air power than we've seen so far."
Among other forces remaining loyal to Saddam are the Fedayeen Saddam -- Saddam's martyrs -- who could number up to 40,000. Saddam's most trusted militia, the Fedayeen have infiltrated regular Iraqi army units, telling them to "fight or be shot in the back," Pentagon Maj. Gen. Stanley McChrystal told reporters when asked to explain the smaller than expected number of surrenders.
U.S. defense officials fear a fight inside Baghdad, a city of 5 million, could lead to both greater U.S. casualties and more deaths among Iraqi civilian men, women and children.
Inside and outside the Pentagon, there also remains the fear of an attack on advancing Americans using weapons of mass destruction.
"It is sort of surprising that they haven't used them yet," Boo said. "If backed into a corner, I think the incentive for Saddam Hussein to use them will shoot up.
Thompson said during Iraq's 1980-88 war against Iran, "The Iraqis have tended to use chemical weapons when in a defensive mode against advancing forces."
U.S. troops have yet to find weapons of mass destruction. Saddam says he doesn't have any, the Bush administration says he is concealing them, and President Bush says Saddam's refusal to give them up was the prime rationale for the war.
There also could be other surprises in the coming days. For instance, Iraqi forces are thought to be laying explosives at principal bridges in Baghdad and its outskirts to detonate if coalition forces arrive, defense officials have said.
U.S. and British officials say they are ready for continued resistance as the coalition vanguards approach Baghdad.
"This is not a video game, where everything is clear and neat and tidy," said Lt. Col. Ronnie McCourt, a spokesman for British forces in the Gulf. "Some enemy who feel that they want to carry on fighting will inevitably do so. We have contingency plans for this. We don't take anything for granted."