After U.S. was attacked by terrorists on September 11, 2001, Bush administration decided to invade Iraq which without United Nations approval. However, the Civil war in Iraq is spreading swiftly. President Bush had promised to help Iraq to pursue democratization, therefore, he sent more troops to Iraq. The grand military expenditure caused American economic fell into a recession. Besides, Iraq war crashed the military families a lot, so the clamor of withdrawing from Iraq had spread. When President Obama took office on 2009, the financial crisis was extending. The whole world was suffered, especially American job loss rates kept rising. President Obama considered that the capital should invest in domestic, rather than outside battlefields. Therefore, he started to withdraw from Iraq. Finally, the last troops withdrew on December 18, 2011. In this case, we understand how important of policymaker in foreign policy.