Barack Obama’s foreign-policy legacy includes reduced American global influence, dramatically underfunded military and intelligence capabilities, and rising concern among longtime allies about Washington’s understanding of international threats. A world of nuclear-weapons proliferation and growing radical Islamic terrorism are the consequences.

There is a reason the world is more dangerous today than eight years ago.

During his White House tenure, Obama regarded national-security policy as a distraction. He preferred instead to concentrate on what he said candidly in 2008 was his main objective: to “fundamentally transform” America. International crises constantly threatened to divert time and energy away from that ideological quest.

This is not to say that Obama did not have his own distinct — and badly misguided — worldview. In Obama’s opinion, and that of all of his top advisors, most definitely including Hillary Clinton, America’s global presence, its strength and assertiveness and its manifest success in protecting its allies and its interests actually contribute to tension, instability and outright conflict.

Under this worldview, American efforts at self-defense and mutual security are part of the problem, not the solution.

Nowhere is the spreading global chaos more apparent than in the Middle East, and it is here that President-elect Donald Trump will face his most immediate international challenges.

In August 1914, British Foreign Minister Edward Grey observed that “the lamps are going out all over Europe, we shall not see them lit again in our life-time.” We are not far wrong to ask whether Grey’s insight applies today in the Middle East:

Radical Islam has spread across the region, shattering governments and leaving anarchy where terrorist groups, warlords and brigands are taking root.

Post-World War I boundaries are disappearing. ISIS has declared a caliphate in what used to be Syria and Iraq.

The Kurds are moving inexorably toward de jure declaration of a “Kurdistan” of uncertain reach.

Turkey is turning away from its secular constitution toward President Recep Tayyip Erdogan’s own concept of a caliphate.

Al Qaeda and the Taliban are resurgent in Afghanistan. Yemen has disintegrated.

Worst of all, Iran is now on a path to deliverable nuclear weapons, legitimized by Obama’s wretched deal, which is providing untold economic benefits to Tehran through unfrozen assets and renewed trade and investment, especially from Europe. Iran’s support for terrorism continues unabated, and its provocative international behavior has only worsened since the nuclear deal. Russia’s influence in the region is higher than at any time since the 1970s.

President-elect Trump has been emphatic that destroying ISIS must be an urgent priority, not Obama’s slow-motion approach that has simply allowed ISIS to continue recruiting adherents and training and deploying terrorists throughout the West. In addition, however, a Trump anti-ISIS strategy must also correct Obama’s misguided reliance on the Baghdad government, which has become little more than an Iranian puppet.

In this complex multi-sided war, the defeat of any combatant inevitably advantages all the others. The goal should be to destroy ISIS while benefiting Iran to the least extent possible.

Obama’s approach, by contrast, seems aimed at enhancing the benefits to Iran.

Indeed, the hardest question of all may be: What comes after ISIS is defeated?

The goal should be to destroy ISIS while benefiting Iran to the least extent possible.

Sunni Arabs who previously supported ISIS (or accepted it because they could not resist) will not again be quietly relegated to the tender mercies of an Iran-dominated Iraqi government or Bashar al-Assad’s regime in Syria.

Either a new state must be created out of the wreckage of Syria and Iraq, or some other durable approach must be found. Moreover, the new Russian airbase in Latakia, Syria, has dramatically changed the strategic environment in the eastern Mediterranean and beyond.

Unfortunately, the base cannot be made to disappear simply by reversing Obama’s erroneous policies.

In the midst of this wasteland that has developed over the past eight years, Israel and America’s Arab friends are desperately waiting for a strong American president who understands who his friends are. President-elect Trump can change the regional political dynamic quickly, signaling that US elections do truly have consequences.

One key step would be to abrogate the Iran nuclear deal in his first days in office. There will be considerable diplomacy required to explain this courageous but necessary decision, but the unambiguous signal it would send worldwide cannot be underestimated.

While terrorism and Middle East anarchy could fill any President’s day, it is critical the incoming Trump administration also fashion strategies to deal with longer-term issues like protecting America’s constitutional system from the advocates of global governance and the realities of international competition from the likes of China and Russia.

Failing to engage in strategic thinking at the outset of any new Administration risks exacerbating the problems that will inevitably flow during its four or eight years in office. Doing the hard preparatory work now will pay off when the uncertain future becomes all too real.

Mr. Bolton is former US ambassador to the United Nations (2005-2006). He is currently a senior fellow at the American Enterprise Institute and the author of “Surrender Is Not an Option: Defending America at the United Nations and Abroad.”