In what way did the war in Iraq significantly alter the outlook for American foreign policy?
The war in Iraq altered American foreign policy largely in that it has made it harder for the US to find allies for itself in Muslim countries. The war, particularly when George W. Bush was president, was very unpopular at least to the general public in many Muslim countries.
The war has also ended up making Iran a bigger focus for the United States. One can argue that the war in Iraq helped to motivate the Iranians to develop their alleged nuclear weapons program. One can argue that they have started to develop it in part out of fear of an invasion for the purpose of "regime change."
Overall, the war in Iraq and the overall war on terror (I have not included the focus on terrorism as part of the effects of the Iraq war because I see the Iraq war as an effect of that focus, not a cause) can be said to have made US foreign policy more difficult.