To understand the Iraq War, you must understand a number of basic facts about the history of the region.
First, you must understand that the US (and a coalition of allies) went to war against Iraq in 1991. This happened because Iraq had invaded and occupied neighboring Kuwait. The US-led forces went to war to force Iraq out of Kuwait but did not try to overthrow the Iraqi government after they had liberated Kuwait.
Second, you must understand that Saddam Hussein, the dictator of Iraq in those times, worked on a program of trying to obtain weapons of mass destruction, also known as WMD, such as biological and chemical weapons. He refused to allow UN inspectors free access to make sure that he was not actually trying to get WMD and that he had destroyed ones that he already had. The US felt sure that, in 2003, he still had WMD that he would not admit to having.
Finally, you must understand that many in the Bush Administration felt that Iraq was in some way tied to the 9/11 terror attacks. They felt that leaving Hussein in power in Iraq would leave the US vulnerable to further attacks.
It was largely for these three reasons that the US invaded Iraq in 2003. The US felt that Hussein was a "loose cannon" who had WMD and might use them to destabilize the region or even to attack America.
The most popular theory is the United States and Britain's explanation on the cause of the war. Which is that it is meant to eventually lead to the disarment of the Iraqi regime, and their weapons of mass destruction. Apperently, this has been their goal for years, but was braught to greater attention after the attacks on September 11th, 2001. Between 1999 and 2001, Iraq rejected a new weapons inspections proposal braught forward by the Bush administration. Although it was never proven, the acts of 9/11 was seen by the U.S, as an act of war. Following these events, the UN and Iraq had begun negotiations on the weapons issue in August of 2002. One month later, the Bush administration called on the UN to take action against "grave and gathering danger" of Iraq. Some weeks later, Britain produced a document which stated Iraq's defiance of the UN's resolution, which led to the U.S' announcement that a massive troop deploymenet to the Persian Gulf region will be carried out in December, 2002. Jumping forward to March, 2003, President Bush gave Saddam Hussein an ultimatum. Which was to leave Iraq which 48 hours, or be faced with war.