Kurt Campbell, who served as the top US diplomat for East Asia during the Barack Obama administration, said the recent flurry of diplomacy with North Korea could set the stage for South Korea's rise.
Trump and Kim are expected to meet before the end of May to discuss the North's denuclearization.
"Ultimately, it puts a huge amount of focus -- the hinge in all of this is South Korea," Campbell told a forum hosted by the Center for American Progress. "South Korea's role and power in deciding the history of Northeast Asia, the future of Northeast Asia, is really on display."
Trump accepted Kim's invitation to meet after being briefed by South Korean officials on their unprecedented meeting with the North Korean leader earlier this month.
The Koreas are also set to hold their own summit in late April.
"What I always encourage South Korean friends in these situations is that you have the ability to play the dominant role here," Campbell said. "What the next year or two could be about really is about South Korea coming of age as a major player in determining their own fate."
The South Koreans, according to the former assistant secretary of state, have always had the sense that the big powers made decisions without consulting them.
"If the president, President Moon (Jae-in), is careful and strategic and puts good effect in place, South Korea has the ability to drive many of the dynamics that are at work now in Northeast Asia," he added.
On what is motivating Pyongyang to engage, Suzanne DiMaggio, who has led informal discussions with North Korean officials, pointed to the regime's progress in its nuclear weapons and ballistic missile programs.
"They understand that their negotiating position is as strong as it's going to be at this moment," she told the same forum, citing the North's tests of a hydrogen bomb and three intercontinental ballistic missiles last year. "So it doesn't surprise me that they're engaging now."
At a separate event earlier in the day, DiMaggio said the US and North Korea could exchange concessions in the lead-up to the summit, even though they wouldn't be described as such.
According to her, the US could downsize joint military exercises with South Korea, which the North views as an invasion rehearsal, while Pyongyang could release the three Americans detained in the communist country.
"I would describe it more as setting the right atmosphere for talks, and I think, if you look at it that way, it makes a great deal of sense," she said.(Yonhap)