[Zon]

Until sometime early in the 1900s (I think just before or after WWI) it was called/named the Department of War.

[APBaer]

The Department of Defense was established by the National Security Act of 1947

[AlGone2001]

Until sometime early in the 1900s (I think just before or after WWI) it was called/named the Department of War.

It was renamed the Department of Defense via the National Security Act of 1947.

This was also when the US Air Force was established, and civilian secretaries were placed over the military brances. There were other changes, of course, but those seemed significant enogh to remember.