What is the primary purpose of the Department of Defense (DoD)?

The primary purpose of the Department of Defense (DoD) is to deter war and ensure national security. This encompasses a range of responsibilities, including the protection of the United States and its interests from external threats. The DoD is tasked with preparing for and responding to military conflicts, maintaining readiness through training and equipment, and developing strategies to safeguard national interests. By focusing on deterrence and security, the DoD aims to prevent conflicts from arising and to protect the country's sovereign integrity.

The other options, while relevant to various government functions, are not the central mission of the DoD. For instance, facilitating international trade and promoting economic development fall under the jurisdiction of other government agencies that focus on commerce and economic policy. Expanding diplomatic relations is primarily the responsibility of the State Department and related entities that manage foreign affairs and international diplomacy. These missions, albeit important, do not define the core role of the DoD, which is fundamentally centered on military preparedness and national defense.