Talks with North Korea are often cast as a tango of two: a dangerous dance between President Trump and North Korean dictator Kim Jong Un. But there is always a third character here, looming in the background. It's China, which makes this whole situation much more dangerous for the Trump administration.

That dynamic was on display on Monday as Trump had talked up the prospect of a second summit between himself and Kim the day before, fueling speculation of possible locations. For Trump, a deal with North Korea emerged as a signature foreign policy goal that, along with his trade deals, could give him a real boost in support in the lead up to 2020. More important than that political maneuvering is the real threat posed, explicitly against the U.S. and our allies, by a nuclear North Korea that Trump is trying to mitigate.

But even as Trump set the stage for next steps in denucularization talks, Kim was on a train headed to China for yet another meeting with Chinese President Xi Jingping, marking the fourth time the two leaders have met in the past 12 months. Indeed, throughout negotiations with Washington, Kim has been backed by China, even flying to the summit in Singapore this summer on a Chinese plane.

The relationship between China and North Korea is understandable. Almost all of North Korea's external trade flows through China, and China has previously supported the dictatorship politically and economically. Moreover, China stands to lose from instability in North Korea, the likely result of economic reforms or regime change, which could send refugees spilling across the border.

Those ties present a real problem for the United States as Trump tries to broker an agreement to denuclearize the Korean peninsula.

The current approach of heavy sanctions on Kim, for example, depends on cooperation from China in restricting cross-border trade. Likewise, cutting the dictatorship's access to nuclear technology also depends on enforcement from Beijing. Indeed, Chinese scientists, through studies and papers co-authored by North Korean counterparts, were recently implicated in sharing sensitive technology.

In addition, China's ties with North Korea could be used by Beijing to gain additional leverage over Washington in seemingly unrelated matters.

It is likely no accident that Kim's meeting with Xi took place simultaneously with a new round of high-level trade talks between China and the U.S. That timing seems to be an implicit threat to Trump that failure to reach an agreement on tariffs could mean more trouble with other foreign policy objectives, including denuclearization.

A warning was also made by Kim during his New Year's address where Kim called for resuming the six-party talks, of which China would be a part, to formally end the Korean War. In that same speech, Kim also threatened to pursue other options should the U.S. continue to call for unilateral denuclearization and continue to impose sanctions. That seems to be an open invitation for more involvement from China, or at least a calculated reminder to Trump that Washington is not the only option, especially if talks with the U.S. continue to yield less than satisfactory results for the North.

Although China has yet to more publicly involve itself in U.S. efforts to push for denuclearization, if the relationship between Washington and Beijing continues to deteriorate, that could easily throw a wrench in any plans — or even an agreement, should China decide to flex its muscles against U.S. interests.

As the Trump administration decides on another meeting with Kim, it cannot forget that China is part of the equation.