At the heart of Saddam Hussein's bid to forestall a humiliating defeat in the days ahead are the Republican Guard and two lesser-known, handpicked units that protect the Iraqi leader and his family.
The cream of Hussein's military are not the best trained or best equipped in the world, and even the loyalty of some among their ranks is suspect. That said, U.S. military and intelligence officials and security analysts believe that the Iraqis could inflict thousands of casualties in urban battles before the war winds down.
"The Republican Guard is absolutely essential to Saddam Hussein's strategy for war," said Gary Samore, a director at the London-based International Institute for Strategic Studies and a former Clinton administration official. "He hopes they'll be able to hold out in Baghdad long enough to create an international uproar that puts enough pressure on Washington and London so he'll be able to survive."
The Americans and British, however, are hoping to peel Hussein's best troops away from their leader by convincing them that there is little future in defending a hated dictator and potential war criminal.
The inner core of Hussein's military is made up of three rings that become increasingly loyal to the dictator as they get closer to him: the Republican Guard, Special Republican Guard and Special Security Organization.
The six divisions of the Republican Guard, with more than 60,000 members, emerged from the 1980-88 Iran-Iraq war and were highly regarded in the region at the time of the 1991 Persian Gulf War.
The Special Republican Guard, whose members have been culled from the Republican Guard and are known for their intense loyalty to Hussein, numbers about 15,000.
The Special Security Organization, the unit closest to Hussein and his family, has about 1,000 members and monitors the other units for signs of disloyalty.
Republican Guard members are paid about 40% more than their counterparts in the regular army, and they are given special housing and automobiles.
During the Gulf War, these troops performed somewhat better than the regular army, though most units fled in the face of the allied onslaught. That said, Mideast analysts add, the retreat was orderly and done in response to Hussein's orders.
"They're my enemy, but I have to give them credit," said Sami Faraj, a Kuwaiti military planner who led reconnaissance missions against the Republican Guard in 1990 and early 1991. "This time, they're also fighting for their survival and their turf."
The regular army probably will serve as little more than cannon fodder aimed at slowing the British and U.S. advance on better-trained troops, analysts said, perhaps by overwhelming the allies with the surrender of thousands of troops.
"I don't think Saddam can depend on the army at all," said Francois Heisbourg, chairman of the Geneva Center for Security Policy and a former senior French Defense Ministry official.
Republican Guard divisions have the best heavy equipment in the Iraqi military. These include most of Iraq's Soviet-built T-72 tanks, which at about 30 years old are the newest in Hussein's inventory. The guard also has a large number of antitank guided missiles.
Analysts said they expected the Republican Guard to avoid battle in open terrain, where nonexistent air support and their hugely inferior tank range put them at a big disadvantage. They might blow up a few bridges or flood the former marshland in the south, but their principal task will be to remain in urban areas.
From their perspective, cities afford a better fight, particularly in Baghdad, given that the Americans and British will be reluctant to kill civilians, thus blunting the allies' superior technology, intelligence and training.
A likely clash could see the U.S. 3rd Infantry Division matched up against the Republican Guard's Medina, Hamurabi and Nebuchadnezzar divisions south of Baghdad, analysts said.
A key consideration for the Republican Guard and the other units in the inner circle will be when they deploy any chemical weapons, analysts said. If they use such weapons too early, Iraq would lose the propaganda war. A more likely point could be after it is clear Baghdad is about to fall.
Ultimately, Hussein hopes to heap so much pressure on the United States and Britain with mounting civilian deaths that the nations are forced to back down, analysts said.
The U.S. has called on commanders of the crack units to defect, urging them to avoid becoming war criminals by refusing to use chemical or other banned weapons.
U.S. forces have dropped leaflets and aimed radio broadcasts at them, even reportedly sending e-mails to generals.
And top administration officials, starting with President Bush, have promised in public comments that these forces will escape punishment if they do not take part in the war.
Some experts, however, criticized the lack of U.S. human intelligence gathering, the last-minute nature of some of these propaganda efforts and an excessive reliance on satellites and other high-tech gadgetry. "Dropping a few leaflets in the final weeks doesn't do it," said Faraj, the Kuwaiti military planner.
Another wild card is the weather. The U.S. appears set to invade with a nearly full moon during a period of frequent sandstorms. "They're launching at one of the worst times of the year," Heisbourg said. "That's a very strange one. I hope this spells confidence rather than overconfidence."
The Republican Guard has spawned rivals for Hussein's affection over the last decade. After the Gulf War, officers in the organization mounted several coup attempts, prompting the dictator to purge its top ranks and cull the most loyal troops for a new organization he called the Special Republican Guard.
Many of the Special Republican Guard members are drawn from Hussein's own tribe and from the cities of Tikrit, Baiji and Al Sharqat. They are the only military troops allowed in Baghdad and have been specially trained in street fighting.
Even closer to Hussein is the Special Security Organization, a security force that is also assigned to protect him and his family. It is overseen by Abid Hamid Mahmud Tikriti, personal secretary to Hussein and his son Qusai, and spends much of its time watching other units for signs of disloyalty.
Analysts say these forces are loyal and widely hated -- a source of motivation given their fear of reprisal should Hussein lose power. Still, some put them more in the category of thugs than good soldiers.
Michael O'Hanlon, a military analyst at the Brookings Institution, estimated that the United States could lose about 1,000 troops if portions of the Republican Guard and Special Republican Guard "fight determinedly."
"This setting is much more favorable to them," he said, "and they may feel they have nowhere to run."
Magnier reported from Kuwait and Richter from Washington. Times staff writer Geoffrey Mohan with the 3rd Infantry Division contributed to this report.