One of the little known factors that influenced the way the Iraq war was fought was careful examination of how the Iraqis operated, and reacted, during the 1991 Gulf War. Interrogations of prisoners and captured documents revealed that the Iraqis were not nearly as quick as American units. Not just quick to move, but in their decision making process. It was also noted that, while the Republican Guard units would often stand and fight to the death, the larger Iraqi army was more prone to flee or surrender if confronted with an aggressive American ground force. Wargames were conducted that took this into account. The officers who played the Iraqi side found that it was extremely difficult to defend with such a brittle force. It was thought that putting the army units into towns and cities might make the troops more willing to fight. But the American players just bypassed the occupied urban areas, and blasted their way through the ones they had to move through. It was also known that the only Iraqi troops with any real training for attacking were the Republican Guard units. So you could move ahead with Iraqi army units on your flanks without too much worry. Out of all this research came the plan for just moving towards Baghdad as quickly as possible. The CIA and Special Forces had developed contacts (via exiles and Arabs in neighboring countries who did business in Iraq) with some commanders in the Iraqi army. It was thought that some Iraqi commanders could be persuaded to quickly surrender, or stay out of the fight, or perhaps even come over to the coalition side.
The Iraqi army reacted as predicted. The new American tactics of just going straight at the enemy and scaring him off the battlefield, worked. Some Republican Guard units survived the bombing attacks, only to get torn to pieces by better trained and equipped U.S. units when it came to a head-to-head fight.
Since Saddam was running a pretty effective police state, with agents and informers everywhere, it was hard to get a sense of how the civilian population would respond. There were lots of exiles in places like Jordan, and other Iraqis, usually smugglers or gangsters of various sorts, regularly moving in and out of the country. But it was discovered that most of these guys were working for Saddam, having been given permission (in return for a share of the profits) to engage in smuggling and other "illegal" activities. Many of the exiles in Jordan were Sunni Arabs, who both hated and admired Saddam. It also became clear that, even in exile, Iraqis feared Saddam. The secret police operated outside Iraq and were not shy about it. Exiles were often approached by Saddams agents and threatened. All exiles had kin back in Iraq, and Saddam was not reluctant to hurt these innocents of their wayward kinfolk did something improper.
One of the major unknowns was how Saddam's core supporters (Sunni Arabs belonging to the Baath Party) would react to the conquest of Iraq. The Sunni Arabs, although only 20 percent of the population, had been at the center of Iraqi politics for centuries. Even though the British had installed a Hashemite (from Saudi Arabia) king in the 1920s, most of the people running the bureaucracy and army were Sunni Arabs. But the monarchy ended in 1958, when Sunni officers murdered the royal family and took direct control of the government. The Sunni Arabs were a rough bunch, well educated and clever as well. Would they accept being a minority? The answer came even before Baghdad fell, as small groups of Sunni irregulars made suicidal attacks on American troops.
The original plan was to have an American division (the 4th Infantry) enter from Turkey and hit Baghdad from the north and south. This, if was thought, would shock the Sunni Arabs into giving up the fight. Instead, the slower advance, just from the south, gave Sunni officials (bureaucrats, Baath party officials, secret policemen) an opportunity to go home and begin planning the next phase of the war.
The United States now had to deal with the same situation that confronted the Turks (when they conquered Iraq in 1534 and drove out the Iranians) and the British (when they drove out the Turks in 1918.) The Turks made a deal with the Sunnis, who then dominated the local government and provided many officers for the Turkish army. The British installed a foreign Sunni Arab (from the noble Hashemite clan of Saudi Arabia) as king of Iraq in the 1920s, but left the Sunnis in charge of everything else. The kingdom of Iraq was actually a constitutional monarchy from the 1930s to the late 1950s, but the Sunnis did not like the idea of being a minority in parliament. Moreover, the Kurds in the north wanted independence, while the Shias in the south wanted to run the country.
Since thousands of Sunni Arabs have decided to fight, despite the opposition of the majority of Sunnis (but support from a large minority), the war won't be over until the armed Sunnis are reduced to the level of scattered bandit and gangster activity. That could take years, and the war plan now appears to be a scramble to put together a police force and bureaucracy that is not dominated by Sunnis, and can defeat the Sunnis who are still armed and fighting. The Shia have not been running things since the 16th century, but it is considered possible for them to do so again. But all this just goes to show you that armies are easier to deal with than politics.