Prediction is very difficult, especially about the future. — Niels Bohr

In the unknown future of US national-security challenges, safe still beats sorry — especially when those challenges already appear daunting.

Which is why the current defense-spending debate — especially the matter of another $500 billion in cuts, beyond the half-trillion dollars’ worth already being lopped off — may be as consequential as any budget decision in recent history.

That’s right: The choices we make about the defense budget, including future weapon systems and troop levels, are fateful; the results will be with us for decades.

Pentagon officials have warned of elevated risks, worried that we’ll no longer be the global power we are today if another $500 billion is slashed from the defense budget, as is planned for year’s end.

Yet some Americans seem to be living in a blissful bubble, oblivious to a world rife with trouble and threats to US security. In some corners, American leadership is not only out of fashion, it’s frowned upon.

But such naivete won’t deal with current problems or prevent crises in the coming years.

Take the Middle East/North Africa. The Iranian threat is hardly diminishing. Few would dispute that Tehran will not only have nukes in a few years, but an intercontinental-ballistic missile to carry them.

Lawless, ungoverned spaces in Yemen and Somalia support the aspirations and activities of terror groups such as al Qaeda in the Arabian Peninsula and al Shabab. Al Qaeda in Iraq is active again and maybe aiding and abetting terror in Syria.

Syria itself? The Assad regime’s survival will certainly find Damascus committed to revenge — and even more wedded to its drive for the bomb.

The promise of an “Arab Spring” has become the reality of an “Arab Winter,” with Egypt, Tunisia and Libya of continuing concern (thanks, especially, to reports of thousands of handheld, surface-to-air missiles missing from Libyan arsenals).

Elsewhere, stability in Afghanistan after our 2014 departure is an open question. A likely security vacuum could leave it vulnerable to the re-rise of the Taliban, al Qaeda and their allies. Instability in (nuclear) Pakistan is good reason for insomnia.

Meanwhile, Team Obama’s strategic shift to Asia will be more of a dainty pirouette than a muscular pivot, absent the forces needed to project US power across the Pacific.

With the military might it’s amassing, Beijing is developing the raw power to challenge Washington for pre-eminence in East Asia — and, perhaps, beyond.

Then there’s North Korea. Not good. Defense Secretary Leon Panetta says, “We’re within an inch of war everyday in that part of the world.”

There’s more. You have to wonder, for example, whether perceptions of US decline made the Russian chief of the general staff think it was OK recently to threaten to pre-emptively strike American missile defenses in Europe.

A world without a strong America is deeply disquieting, one where US interests will be under assault from a number of different directions, including cyberspace.

Fact is, as Sen. Jon Kyl (R-Ariz.) noted last week at The Heritage Foundation, “We don’t always have the luxury of deciding where and when we will have to confront evil in the world.” The 3 a.m. phone call is a come-as-you-are affair; we better be ready.

Thankfully, some in Congress such as Kyl and Rep. Paul Ryan (R-Wis.) are working to stem America’s slide to second-rate-power status by finding ways to trim the federal budget that shift the heavy reliance away from defense-spending cuts.

This is no time to give short shrift to American security. We need a hard-nosed foreign policy backed up by a strong defense. The past shows the painful consequences of being weak and ill-prepared.

Threats will develop irrespective of our ability to defend ourselves. Wishful thinking about a future no one can accurately predict is no foundation for a national-security policy.