Tuesday's summit in Singapore between President Donald Trump and North Korean leader Kim Jong Un should be a viewed as a positive development but is unlikely to be a "momentous event," said Kirk Wagar, U.S. ambassador to Singapore under former President Barack Obama.

"This is a very good thing," Wagar told CNBC's "Squawk Box" on Monday. But he said that any past U.S. president could have gotten a one-on-one meeting with the leader of North Korea.

"The issue is, of course, is that the stated foreign policy goal for almost 30 years has been a one-on-one with meeting with the United States president to elevate their stature," he said.

There have been failed diplomatic attempts by U.S. presidents for decades to stifle North Korea's nuclear ambitions. Trump's predecessor, Obama, turned up sanctions in an unsuccessful bid to force Pyongyang to the negotiating table.

Wagar told CNBC that until Trump, the recent U.S. position had been "we will have six-party talks that included Japan, [South] Korea, China, the United States, and North Korea … [and] Russia was the other one." That's why direct talks with North Korea never happened, he said.

It's not a matter of giving or denying Trump "credit" for the meeting but about results, Wagar said. "I don't think we're going to see a momentous agreement." But he predicted the meeting could be the beginning of a discussion that "puts us on a diplomatic path."

The Trump administration's desire for a win "increases the chances that this goes forward, which I think is a very positive moment," Wagar said.

Secretary of State Mike Pompeo on Monday said the Singapore summit is "truly a mission of peace," adding the U.S. is "eager" to see if Kim is "sincere" about denuclearization. "If diplomacy does not move in the right direction ... [sanctions] will increase," Pompeo warned.

Going into the meeting — which is set for 9 a.m. local time in Singapore (9 p.m. EDT Monday) — the president should remember that North Korea only looks out for itself, Wagar said.

"Given the fact that more information is coming from the outside world into their population, I think the worry of the regime is always going to be whether there's going to be an uprising that knocks them out of power," he said.

"The nuclear testing has got them to the point where they now get this meeting," he said. He added that he believes the White House is correct in saying that "'this is a unique moment in history.'"

Scott Snyder, a U.S.-Korea policy specialist at the Council on Foreign Relations, told CNBC earlier on Monday that the summit is "essentially the normalization of Kim Jong Un and North Korea on the international stage."

The Trump administration's challenge will be to make sure the U.S. is not normalizing the isolated country as a nuclear weapons state, Snyder added.