Kim Jong-un's second visit to China in 40 days, following his March 28 meeting with President Xi, demonstrates that the two leaders continue to work together as closely as “lips and teeth,” a metaphor long used to describe the relationship between North Korea and China.

Why it matters: With U.S. pressure mounting, Kim needs to ensure China’s support before meeting with Trump, while China wants to show that it is still a key player. The two leaders doubtless exchanged views and coordinated strategy.

This visit occurred right before Wednesday's planned trilateral summit between China, Japan and South Korea in Tokyo, where North Korea is expected to be a central topic of discussion. It also comes two weeks ahead of President Moon’s White House visit and a month before the much-anticipated Trump–Kim summit.

The meeting may help to explain why the White House has not yet announced the date and location of the Trump–Kim summit, despite Trump’s statement that details are decided and the announcement imminent.

Trump has pressed China on trade, but needs its support in dealing with North Korea, posing a policy dilemma. It would be a mistake for Trump to think he can strike a nuclear deal with North Korea all by himself. Chinese involvement is inevitable; thus, it makes sense to bring China into the process early on, rather than deal with complications arising from a later intervention.

The big picture: Kim is clever, strategic and not to be underestimated. And China can't be expected to sit these events out — it will do everything it can to remain a key player in the negotiations.

Gi-Wook Shin is chair of Korean Studies at Stanford University, director of the Walter H. Shorenstein Asia-Pacific Research Center and senior fellow at the Freeman Spogli Institute for International Studies.