It has been nearly three weeks since President Donald Trump surprised the world with his "on-the-spot" acceptance to meet with North Korean leader Kim Jong Un by the end of May, and the White House is scrambling to nail down the details.

Aside from a proposed agenda to discuss the North's nuclear program, the White House has yet to produce a date, time, location or a list of Trump administration participants.

Adding to the disarray, Kim has been suspiciously silent, at least in public — and it has cast a shadow of doubt over the potential meeting. What's more, Kim traveled on a secret train trip to Beijing, which concluded Wednesday, but even that was shrouded in mystery.

Some experts view Kim's visit, the first known journey abroad since he assumed power in 2011, as preparation for upcoming summits with the U.S. and South Korea.

However, the North's history of breaking promises is all too familiar to the last American diplomat to successfully secure a deal with North Korea — only to see it fall apart a few years later.

"The South Korean National Security Advisor is saying that Kim Jong Un is prepared to meet with President Trump AND give up his nuclear weapons if the meeting goes well," former U.S. Ambassador Robert Gallucci, who is now chairman of the U.S.-Korea Institute and a Georgetown professor, wrote in an email to CNBC. "One would think that the North would have a comment."