Usually, when you sign up for a new service, such as internet, phone, or television, you are required to sign a contract. It usually contains a commitment to pay for that service for a set period of time, and breaking that commitment requires a heavy fine. In many ways, war is a contract.
When we invaded Iraq, with the huge support of the American government, and the American population (support levels over 70%), we signed a contract to complete the mission. We invaded Iraq with the goals of removing Saddam Hussein, Removing WMDs, Installing a Democracy, and Helping the new government to able to stand on its own. Those were the reasons we invaded. Our contract in this war, like all our other wars, was to complete the mission.
If we fail to fulfil the terms of that contract, then we will also face a heavy fine. However, unlike simply paying a monetary fine, our price for not completing the mission in Iraq will be much greater. If we fail to complete the mission in Iraq, the cost could very well be a regional war, more terrorist attacks in America, and a defeat for peace and freedom around the world.