The Iraq War was started as part of the War on Terror, there are actually two parts to this.
Operation Iraqi Freedom, which is focused on the Taliban infested Government of Iraq, and Operation Enduring Freedom which is focused on hunting down and eliminating Osama Bin Laden and the Al-Qaeda in Afghanistan.
Basically the glue that ignited both wars are the Taliban, think of it this way:
Al-Qaeda attacks US,
US counters with military search in Afghanistan,
Taliban attack us while intervening,
We switch attention to their main mass in Iraq, protected by the oppressive government,
We see travesties caused by Hussein,
We respond with a full military overthrow.
It is not an oil war like liberals spread it to be, the oil is in the other Middle East countries, only a small fraction is in those two. Take it from people who have been there.