(CNN) If there's a single message of President Donald Trump's foreign policy, it's that Trump can do better than previous presidents and world leaders. Just trust him.

Put another way, the Trump doctrine so far is: Walk away; I can do better.

In May so far he's walked away from his own planned and much-ballyhooed agreement to sit down mano a mano with the North Korean dictator Kim Jong Un after a North Korean official insulted Vice President Mike Pence.

This may have been an excuse; Trump, while recently deferential to Kim, despite his reputation as a murderous strongman, has doled out his share of insults in the past.

But it's a maxim he's lived by.

"'Know when to walk away from the table.' The Art of the Deal," he once tweeted.

Walking away can be harder, however, when the stakes are nuclear and the world is nervously watching.

"The art of diplomacy is a lot harder than the art of the deal," Sen. Bob Menendez told Secretary of State Mike Pompeo on Capitol Hill after news that the North Korea summit was canceled broke.

Trump has shown a willingness to walk away from plenty of things as President.

He also chanced Mideast upheaval and walked away from the carefully crafted but fragile international pact with Iran, saying it was "disastrous" and "defective at its core," and complained that it gave an untrustworthy regime billions in sanctions relief. (We won't try to square that with Trump's promise that North Korea " will be rich " if Kim would only come to the talks in Singapore.)

Declaring his intention to pull out of the Iran deal, technically the Joint Comprehensive Plan of Action, Trump called it "a horrible, one-sided deal that should have never, ever been made."

He could do better if only Iran would come to the table with him.

"Great things can happen for Iran," he said.

As proof of his word, he pointed to North Korea.

"We think relationships are building with North Korea," he said. "... It can be a great thing for North Korea."

Great things for Iran and great things for North Korea -- in one appearance.

It's all about the relationships for Trump. In his mind, he's waiting to make the deal. Or to walk away.

Announcing the cancellation of the summit with North Korea Thursday, Trump sent a very personal-feeling and oddly respectful letter directly to North Korean leader Kim Jong Un. CNN's Jim Sciutto called it a "nuclear Dear John."

And it was clear the door is open, in Trump's mind, for renewed talks.

"I felt a wonderful dialogue was building up between you and me, and ultimately, it is only that dialogue that matters. Some day, I look very much forward to meeting you," he wrote.

JUST WATCHED Hear Trump's letter to Kim Jong Un Replay More Videos ... MUST WATCH Hear Trump's letter to Kim Jong Un 01:50

His language was not as friendly, but he made a similar entreaty to Iran.

"Iran's leaders will naturally say that they refuse to negotiate a new deal," he said. "They refuse, and that's fine. I'd probably say the same thing if I was in their position. But the fact is, they are going to want to make a new and lasting deal, one that benefits all of Iran and the Iranian people. When they do, I am ready, willing, and able."

In fact, Trump has promised to pursue a variety of new deals as he's pulled out of old ones.

Announcing the end of US involvement in the Trans-Pacific Partnership in a memorandum, Trump said, "it is the intention of my Administration to deal directly with individual countries on a one-on-one (or bilateral) basis in negotiating future trade deals."

Announcing the end of US involvement in the Paris Climate Agreement , Trump said, "So we're getting out. But we will start to negotiate, and we will see if we can make a deal that's fair. And if we can, that's great. And if we can't, that's fine."

Canceling the North Korea summit is something very different and it seems he'll apply the same poker mentality -- fold when you need to -- to his own potential deal, for now.

He's threatened, similarly, to pull out of NAFTA if it can't be restructured and he's complained about NATO.

The problem for Trump is that he's now pulled out of existing deals and put the brakes on his own. That leaves the President who wrote "The Art of the Deal" with something of a deal deficit as we approach the halfway mark of his presidency.

North Korea could come back. Iran could come back. A China trade deal could work out. NAFTA could get renegotiated. If so, great. As Trump said recently of the North Korea summit, "We'll see how it all works out. Maybe it will. Maybe it won't."

He wasn't kidding.