A more thorough explanation:

The Defense Department is a part of the United States government that is responsible for coordinating and overseeing military affairs and agencies responsible for national security. It was established in 1947 as the National Military Establishment and was later renamed the Department of Defense in 1949. The Department is made up of the Army, Air Force, Navy, Marine Corps, and Joint Chiefs of Staff. The Secretary of Defense is the head of the Department and is answerable to the President as Commander-in-Chief.

When the United States goes to war, it is the Defense Department that plans and executes military operations. For example, during the Gulf War in 1991, the Defense Department coordinated the efforts of the Army, Air Force, Navy, and Marine Corps to liberate Kuwait from Iraqi forces.

Another example is the Defense Department's role in protecting the country from cyber attacks. The Department works with other government agencies and private companies to prevent and respond to cyber threats that could harm national security.

These examples illustrate how the Defense Department is responsible for protecting the United States and its interests both at home and abroad.