The national security doctrine that led to the US invasion of Iraq in 2003 is popularly known as the Bush Doctrine. The aspects of this doctrine that were most relevant to the Iraq war were the idea of preemptive war and the idea of unilateralism.
This national security doctrine held that the United States had the right to attack countries or groups that we think might threaten us. We do not have to (and really should not) wait until they truly are a threat. Instead, we should make sure they never become a danger to us.
Secondly, the doctrine held that the US does not need the blessing of international bodies or of any allies in order to act. Even though the UN did not sanction our invasion, we decided that it was right to invade.
These are the two aspects of this national security doctrine that are most relevant to our decision to invade Iraq.