That's a common misconception. The DoW was just the army, and the navy was a separate cabinet department, until the National Military Establishment was created in 1947, and that department, not the DoW, was renamed to the DoD in 1949.

The DoW became the departments of the army and air force, inside the DoD.

See third paragraph in that Wiki link, and second paragraph in this one:

https://en.wikipedia.org/wiki/United_States_Department_of_th...