This study investigates the root cause of a perceived lack of unity-of-effort - the absence of an effort to coordinate elements of the State Department, the Defense Department, and other government agencies to combine capabilities to achieve unified strategic efficiencies - in US foreign-policy execution. The author evaluates different potential explanations for the source of the issue, and analyzes their validity by reviewing the relevant Congressional law that regulates the State and Defense Departments. The author concludes that prevalent explanations of the issue are merely symptoms of the root cause. The author attributes the perceived unity-of-effort problem to the way in which Congress has regulated the State and Defense Departments by law, and reformed their roles in US foreign-policy execution. This Congressional legislation appears to have developed as a result of the United States' emergence from World War II as a hegemonic superpower. The author also concludes that the two pivotal laws, the Foreign Service Act of 1980, and the Goldwater-Nichols Department of Defense Reorganization Act of 1986, in combination with the end of the Cold War and the development of the Global War on Terrorism, exacerbated the unity-of-effort issue.