This is a question with many possible answers. In fact, in the winter of 2003, I was teaching International Relations and I structured my whole class around the idea that the US was going to invade Iraq and that we, as a class, needed to understand why. We used the various reasons for invading Iraq to look at various theories of international relations.
The major reasons for invading Iraq were:
- The belief that Iraq had weapons of mass destruction.
- The belief that Saddam Hussein would use those WMD in ways that would destabilize the region.
- The fact that the region is vital to both the American economy and the economy of the whole world (because of oil).
- The belief that Iraq had been involved in the 9/11 terrorist attacks and might help with further attacks.
- The desire to rid the region and the world of a brutal dictator.
All of these were major reasons cited for invading Iraq.