‘I’ve got a pen and I’ve got a phone” was President Obama’s favored path to a foreign-policy legacy. Oops. President-elect Donald Trump may now use his own pen and phone to erase it.

But it won’t be that easy.

At first glance, the Paris agreement on climate change, the Cuba détente and the Iran nuclear deal seem easy to undo. Like many of Obama’s stabs at history-making, they were done unilaterally, without congressional advise and consent.

But Obama’s pen didn’t quite write his ticket to immortality in disappearing ink.

Take Iran. Obama and Secretary of State John Kerry insisted that because the Joint Comprehensive Plan of Action wasn’t a “treaty,” there was no need for Senate approval. After months of grueling negotiations, it was sealed with a few phones calls and the stroke of a pen.

The deal, however, does carry legal weight. In July 2015, the UN Security Council passed a resolution endorsing it and calling on all UN members to implement it. As far as our allies are concerned, a deal’s a deal.

Yet, Trump isn’t merely stuck with this raw deal, either. He has options.

For starters, he can simply insist on its compliance. Just this week the International Atomic Energy Agency reported that Iran is storing more heavy water than the deal allows. Until now, Obama called such violations “marginal.” We even bought Iran’s excess heavy water to help it comply with the deal.

“The water that John Kerry has been carrying for Iran is enough to fill many oceans,” says David Ibsen, president of the advocacy group United Against Nuclear Iran. Trump, he added, will likely be less forgiving.

And rather than going to the United Nations to resolve each dispute over violations, Trump can unilaterally impose new American sanctions, which could turn the screws on Iran’s economy once again.

The Obama administration has been encouraging American companies to enter Iranian markets to assure the deal’s viability. Ibsen’s organization, however, works to deter them from doing so. Friday, Veterans Day, UANI plans to mobilize veterans to remind some of America’s largest corporations that Iran is responsible for the deaths of 1,000 American soldiers, and that it’s a risky place to do business.

Trump can follow suit, bleeding the Iran deal to death with cut after cut to its participation in the global economy.

And the same goes for Cuba. Obama, as part of his deal with the Castro regime, unilaterally removed Cuba sanctions. Trump can now reinstate them, fulfilling a promise he’s made to Floridian voters of Cuban descent.

Then there’s the climate pact. Like the Iran deal, the Paris agreement — which Trump has said he opposes — carries the weight of mutual obligations among its signatories. But its carbon-emission restrictions have no enforcement mechanism.

Trump can simply stop abiding by them with no serious consequences beyond some tsk-tsking in European capitals. (Not to mention the fact that Trump’s plans to boost the economy by unleashing American innovation is more likely to reverse harmful climate trends than international rules few will follow.)

Meanwhile, diplomats from allied Asian countries tell me there’s much anxiety over Trump’s promise to end the Trans-Pacific Partnership trade deal (designed in part to blunt China’s economic rise). A promise to cancel NAFTA similarly frightens Mexicans and Canadians in this hemisphere.

Yes, some suggest Trump may renegotiate marginal provisions of those trade deals to improve them without deep-sixing them. If so, free global trade will survive.

So, true: Undoing Obamaism, whether advisable or not, will require much more finesse than Trump gave it on the campaign trail. But remember, Obama came into office believing he had a mandate to undo some of his predecessor’s most controversial foreign policies. So will Trump.

There’s a lesson here: If Trump wants his presidency to be remembered for its impact on policy, he’ll need to use his art of the deal to bring Congress along on them. Excluding Congress was how Obama put such deals at risk to a Republican successor.

And there’s some irony for Obama: He was swept into office denouncing his predecessor’s supposed “imperial presidency.” In the end, he tried to increase executive power himself.

He wanted to be “transformative” and, to make it easier, he acted unilaterally. But he also made it easier to undo his legacy. With a phone and a pen.