For whatever reason Donald Trump’s supporters voted for him — because he’s anti-establishment, because they liked his unpolitical tell-it-as-I-see-it approach, or because they simply didn’t like Hillary — one of the reasons was clearly not his foreign policy expertise.

The US (and obviously our ally South Korea) faces one growing threat: North Korea, which is well on its way to having the capability to strike the US mainland with nuclear-tipped missiles. Which raises an obvious question: is Kim Jung-Un doing this for defensive or offensive purposes? And the equally obvious answer: defensive.

North Korea fears — from our perspective a totally irrational fear — that the US might act aggressively against it. But looking at it from their perspective, perhaps it’s not so irrational. After all, the North Korean regime was included in George W. Bush’s “axis of evil’’ over 15 years ago.

And Donald Trump has just upped the ante. What purpose was served by his speech Tuesday, before dozens of heads of state at the annual UN General Assembly meeting, in which he threatened to “totally destroy North Korea,’’ while referring to the regime as “a band of criminals’’ led by “Rocket Man.” (But cut our leader some slack: at least it wasn’t “Rocket Boy.”)

Might Kim read the threat as a realistic one? Might he therefore accelerate his nuclear missile tests? Does Trump believe that such threats as he delivered lower the tension on the Korean peninsula and reduce the likelihood of a military escalation?

Nor, of course, was North Korea the only country Trump threatened in his bombastic, rambling UN address.

He described the Iranian regime as a “murderous” and “reckless’’ one, though his clearly stated threat to abrogate the nuclear deal President Obama struck with Iran is what seems really reckless. Calling the deal “one of the worst and most one-sided transactions” the US has ever undertaken — “an embarrassment’’ for our country — Trump marched defiantly down a path with but three possible outcomes: get a better deal from Iran, swallow his pride and live with the “embarrassment,” or renege on the deal.

The first option is surely a non-starter: Iran has made that quite clear. And it’s hard to believe Trump intends to live with this self-described “worst and most one-sided” transaction. So option three — unilaterally walking away from the deal — is clearly what Trump has in mind.

His views on Iran no doubt reflect his visit to Saudi Arabia a few months back as well as frequent conversations with Israel’s Prime Minister Netanyahu. Saudi Arabia and Israel don’t see eye to eye on much, but they are joined at the hip in their anti-Iran focus. Indeed, they’d both like nothing better than a war with Iran — one, that is, that the US fights. And they’ve clearly got Trump firmly — if, at least, not that aggressively — in their camp.

This is hardly news: the US and Iran have not exactly had warm relations since the Shah was overthrown more than 40 years ago and some 50 American diplomats were subsequently held hostage for over a year.

But our relations improved significantly under Obama with the signing of the nuclear deal two years ago. The agreement between Iran and the five permanent members of the UN Security Council — the US, Great Britain, France, China, and Russia — basically put Iran’s nuclear ambitions on hold for 15 years. And there is zero evidence that Iran is not fulfilling its legal obligations.

Right-wing conservatives — whose lead Trump is following — were outraged by the deal from the beginning, though it’s hard to understand why. Getting Iran to forsake its nuclear ambitions would seem well worth the lifting by the US of various economic sanctions — which had been placed originally because of Iran’s nuclear ambitions.

Iran is a country of 80 million people, larger than Iraq, Syria, and its Arab Gulf neighbors combined. It’s by far the largest Shia country in a Muslim neighborhood dominated by Sunni Muslims. Only Turkey is a serious rival to Iran. Saudi Arabia, its key Arab antagonist, is basically a one-pony power: oil. And Saudi Arabia’s global significance, like the price of oil, has definitely peaked.

Meanwhile, Iraq, 15 years after the American invasion, is still a weakened shell of what it was under Saddam Hussein, its colonial border enclosing three separate entities: the oil-rich Shia area in the south, Sunni-dominated Baghdad and its environs, and the Kurdish north. The Kurds want independence, and indeed will vote for it in a referendum next week, which will only further destabilize the situation.

Syria remains a disaster, though its beleaguered president Bashar al-Assad has managed, with Russian help, to retain power. Syrian Kurds, whose soldiers the US has depended on in its battle against ISIS, have essentially carved out a small portion of northern Syria near the Turkish border, which infuriates the region’s only stable nation, our NATO ally Turkey. In short, the Middle East, eight years after the Arab Spring was proclaimed, is a volatile, turbulent region.

Trump is right in one regard: Iran is indeed part of the problem — or, anyway, of our problem. It supports Syria’s Assad; in Lebanon, it’s aligned with Hezbollah, which the US considers a terrorist group; in Iraq, its influence is greater than that of the US. From the US perspective, the Iranian nuclear deal was not perfect. How could it be: it was a negotiated compromise. But, by getting Iran to put a seal on its nuclear hopes, it assured that Saudi Arabia would not develop nuclear weapons. Keeping the Middle East de-nuclearized was, and is, a major accomplishment.

How would Iran react were the US to renege on the treaty and punish it with ever-more stricter sanctions? It would only make Iran more aggressive; it could even encourage them to resume their nuclear development. It would certainly further destabilize what is probably the least stable region in the world.

And how will it reflect on US credibility worldwide?  More specifically, if you were North Korea, with an established nuclear capability, would you want to make a deal, giving up that key defensive protection, with a deal-breaker?

What is Trump’s purpose?