Wasn’t President Obama’s Iran nuclear deal — what he considers his top foreign-policy achievement — supposed to mellow the mullahs?

If so, it’s failing. Badly. As Washington’s transition to President Trump fast approaches, Iran is acting increasingly aggressive.

Secretary of State-designate Rex Tillerson was right when he told the Senate Foreign Relations Committee Wednesday that under Obama, America was all too willing “to ignore violations of international accords, as we have done with Iran.”

Take Tehran’s habit of producing more plutonium than the deal allows. We’ve already learned that the Obama administration helped Iran avoid this violation by simply buying — in some cases, with bags of cash — the excess heavy water that Iran was overproducing.

US officials offered justifications for their actions, of course. For example, they argued, by getting heavy water out of Iran, we’d be slowing its path to a bomb. And that’s worth the cost of bribing the mullahs a little, no?

Except there’s more than monetary bribery involved. This week the Associated Press reported that in addition to some $8.6 million that the United States paid Iran to ship out its excess heavy water, we also gave Iran other compensation — in the form of 116 metric tons of raw uranium.

Now, it’s true that under the nuclear deal Iran is allowed to possess raw uranium, which can be used for peaceful energy production. But if further enriched, it can be used to make bombs. In this case, the raw uranium we gave Iran, once enriched, could make up to 10 nuclear bombs.

And as former UN nuclear inspector Olli Heinonen reports, Iran is acquiring raw uranium from other sources as well. Which, as Heinonen says, “suggests [Iran] may be stockpiling uranium to reach nuclear breakout before the deal’s initial limitations expire.”

So what happens if America under President Trump decides to tear up the deal? Now, thanks to Obama’s help, if the mullahs make the likely decision to resume their mad dash to producing the bomb, they’ll have all the raw material they need.

It almost sounds like we’d be foolish to break the deal, because that would ensure Iran’s fast-track path to a nuke. Obama’s insurance against his deal being undone was, in effect, to make the whole situation much more dangerous.

Tillerson said that, if confirmed, he’d order a “full review” of the Iran deal. There’ll be “no nuclear enrichment in Iran, no storing of nuclear materials in Iran.” And we’d “hold [the Iranians] to that agreement.”

But with all our secret and known deals, it’d seem difficult for President Trump to undo President Obama’s legacy project.

And there’s another concern: Would Iran risk a military confrontation with us? After all, Obama & Co. have long argued that the deal — and the improvement in Iran’s economy it entails — will strengthen Iranian “moderates” and weaken the hardliners.

Yet since the deal was struck, Iran has become more aggressive, sending weapons and fighters to Yemen, Iraq, Lebanon, Syria and the rest of the region increasingly destabilized by Iranian terror proxies. This week, a new UN report documented Iranian weapons transfers to Hezbollah in Lebanon, the Houthis in Yemen and other nefarious players, in violation of a UN Security Council ban on such Iranian exports.

Worse: Iranian boats this week got dangerously close to an American destroyer, the USS Mahan, in the Strait of Hormuz. The Mahan was forced to fire warning shots, risking a major confrontation in a busy commercial naval passage.

Plus, this week Akbar Hashemi Rafsanjani, a rival of Supreme Leader Ayatollah Ali Khamenei, died. Iran watchers tell me that’s good news for Khamenei’s allies, the Revolutionary Guard, which is now set to further dominate Iran’s politics.

The Guard already controls much of Iran’s economy and backs an extremely hard line against America while advancing the clandestine nuclear program.

So what should the incoming administration do?

Tillerson indicated that his preference would be to negotiate a better deal. One way to do that is to designate the Revolutionary Guard as a terror organization.

Another powerful signal we could send: Recommit to ridding Persia of the clerical regime that has wreaked havoc on the country and its neighbors.

By doing so, we’d put it on notice: Make the wrong move, and you risk confrontation with the world’s foremost superpower. At least Washington will no longer bend over backward to help you break the rules.