The war in Iraq was not declared as the Constitution demands.
From the Constitution:
"ARTICLE 1, SECTION 8
The Congress shall have Power:
To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;
To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;"
It is clear from the first part that it is the Congress which may initiate war. Obviously, the president does not need permission to defend, but invading sovereign nations requires a declaration of war. A declaration of war is very important for it lays down the moral justification for the conflict. Such justification is for the international and domestic communities. It spells out why you are left with no other option, but to murder citizens of another nation.
Another purpose of a formal declaration of war is to unite the nation for the sacrifices that they will need to endure to be successful. A declaration of war is not to be taken lightly for it is literally gambling the entire nation. The case of war is clear when the survival of the nation is in question. For all other conflicts, it is even more important that everyone is in agreement. They will pay the bills, both in blood and money. Disunity in the midst of a war is not good. It is better to hammer out the need for the war in an official forum before any conflict. The key point here is the before part, because the failure to do so will create disunity almost immediately upon entering any conflict.
When a war is not in the best interest of the people, the President must end it. If, however, he chooses to continue a conflict, it is the second portion from above that offers the solution. Congress, by denying funds may unplug the war machine. I have to make it clear that having the President end the conflict is the preferred choice for that will allow the best options for diplomacy and a controlled removal of troops.
That said, when the President fails to obey the will of the people, Congress must act. With control of the purse strings, Congress has the power to terminate any conflict. It may not be pretty or well managed, but it is the moral duty of Congress to fulfill the will of the people.