How can the frustration-aggression hypothesis be used to explain US involvement in Iraq?
This theory holds that aggression is caused by frustration. When our plans are thwarted and we cannot attack the person that is truly frustrating us, we take our aggression out on others.
In the case of Iraq (you can argue), the US took its frustrations out on Saddam Hussein when it started to lose its place as the dominant power in the world. You could argue that this trend started as China rose and as North Korea and Pakistan got nuclear weapons and came to a head when Al-Qaeda attacked on 9/11. These things made the US feel weak and frustrated. You can argue that the US could not do anything about the actual problems and so attacked Iraq instead.
This would be a way to say that US involvement in Iraq is congruent with the frustration-aggression hypothesis.