The relevance of DoD has declined steadily since the end of the Cold War. Coming to grips with its passing won't be easy, but the Navy is working through the five stages of grief and toward a future in cyberspace.
First, the unpleasant truth: the Department of Defense's raison d'?tre died with the Cold War. No one likes to talk about it, but that's what happened. Created in the National Security Act of 1947, DoD is wholly a creature of what eventually became the United States' hair-trigger during the nuclear standoff with the Soviet Union. Prior to that, we basically stuck to the Constitution's mandate to "provide and maintain and Navy" on a constant basis and to "raise and support Armies" as the situation demanded.
The Cold War's odd combination of non-war (we never fought the Soviets) and non-peace (we constantly mixed it up in proxy conflicts and arms races) forced the merging of our republic's two historically distinct security roles: