President Donald Trump is the wildcard when it comes to the potential summit between him and North Korean leader Kim Jong Un, according to a geopolitics expert.

"It is not clear what the calculus is with (Trump)," Lanhee Chen, a research fellow at Stanford's Hoover Institution, told CNBC's "Squawk Box" on Tuesday.

"We know that there are issues that trigger the president. But we don't know exactly what the end game is," he said, adding that the question of legacy could be very tempting to the president.

In comparison, North Korea's Kim is seen to be the more predictable leader, Chen said.

"Kim Jong Un behaves in a certain way rationally," the Stanford expert said. "We know (Chinese President Xi Jinping) is influential, domestic politics are influential, or maybe he wants an exit strategy. But with President Trump, it is more difficult to tell."