The most well-known perspective in International Relations is realism. Realism holds that countries act in ways that are calculated to increase their own power. They do not worry about things like morality or international law. From this point of view, the United States intervened in Iraq to protect its own power. It did so because it felt that Iraq was a threat to American security via the weapons of mass destruction that it supposedly had. Alternatively, it felt that Iraq was a threat to the stability of the oil-rich Middle East.
A second perspective in International Relations is liberalism. It holds that peace can be maintained in the world by international laws and by more countries becoming democratic. From this perspective, the US and Iraq went to war largely because Iraq was not democratic. The fact that it was ruled by a dictator made in unstable and likely to act in ways that would cause war. The US also intervened to uphold international law such as the orders of the UN.
A third perspective is a more radical, Marxist position. This perspective emphasizes class conflict. From this perspective, the US is the dominant power in the world. It has the strongest economy in the world. It wants to ensure the strength of its economy and its ability to exploit other countries. From this point of view, the US went to war to ensure its world hegemony. It needed to make sure that it would be able to continue to exploit the resources of Iraq and the surrounding area.