Prior to 1947, the United States Government divided military duties between the Department of the Navy and the Department of War. That year, the two Departments were consolidated into the National Military Establishment, and the Cabinet positions were merged into the Secretary of Defense.
Two years later, and in part due to the pronunciation of the NME acronym, it was changed again to the Department of Defense.
Think about how you feel when discussing a "Department of Defense", or perhaps the even more anodyne "DoD". Consider how the word "Defense" positions itself in your mind. For me, it conjures a primarily benign image, somewhat passive, deliberate. It feels more ponderous, like something you need to thwack with a stick a couple of times to get its full attention.
Now rally up a thought or two on a "Department of War". Again, for me, it's aggressive, menacing, focused, waiting to pounce. Not a child's toy, and certainly a Cabinet-level Department that requires close watch. A dangerous weapon.
And just for fun, what does the "Department of Homeland Security" feel like? Slight undertones of Nazi-era rhetoric along with peak notes of strangled bureaucratic mess and pure committee hell.
Defense => War.
Homeland Security => Defense.
Let's be honest with ourselves, especially in matters of our Nation's safety, and the lives and deaths of her citizens and soldiers. We have a standing military that is the world's most powerful and lethal war-making machine. It consumes nearly 1 out of every 5 Dollars our government spends. Let's call it by its true name, the Department of War, and let's deal with the fundamentally serious task of how to maintain and use it with extreme candor.
No more armed conflicts, no more resolutions on the use of force. Only Congress has the Constitutional authority to declare war on a foreign entity, and that's where that power shall reside.