I am a child of the Cold War. As such, my thinking for decades was conditioned by the great issue of that era: How to maintain freedom in the face of our perceptions of Soviet ambitions for world domination?
For the first few decades of the Cold War, the United States strategy for achieving this objective was containment backed up with a powerful nuclear deterrence. But as the nuclear arms race heated up, it became increasingly clear that this strategy risked precipitating a nuclear holocaust. Thus, by the late sixties, nuclear arms control had become the overriding security issue - certainly it dominated my thinking on security during that era.
But with the ending of the Cold War, the threat of nuclear holocaust receded and arms control, as we had practiced it during that era, was no longer the dominant security issue. The most serious threat to the United States became nuclear weapons in the hands of failed states or terrorists - used not in a standard military operation, but in extortive or apocalyptic ways. Therefore, in the present era, preventing the proliferation of nuclear weapons replaces arms control as the organizing principle for our security. Certainly it has dominated my thinking on security for the last decade.