What was the main purpose of the department defense?

The main purpose of the Department of Defense (DoD) is to ensure the national security of the United States by coordinating and overseeing the nation's military forces. It is responsible for defending the country against external threats, conducting military operations, and supporting international alliances and partnerships. The DoD also plays a crucial role in military strategy, defense policy, and the management of defense resources. Ultimately, its mission is to protect the nation's interests and maintain peace and stability.