because most americans do not want war, at least id hope, so calling it that seems pretty short sited (maybe until you continually do that 'war' thing), if you want the citizens to look positively on your spending it should probably be for defense not war, again, at least i should hope. im just a dumb "lib" whatever that means
On the other hand calling it "Department of Defense" seems quite whitewashing of what it actually does.