China and North Korea boast an alliance forged in blood -- more than 130,000 Chinese troops, including the son of Mao Zedong, died defending the North during the Korean War -- but the relationship has always been an uneasy one.
While Pyongyang is dependent on Beijing for trade and diplomatic support, experts say the North Korean regime has always resented playing the little brother role to the much bigger China.
Now, as North Korean ruler Kim Jong Un establishes himself on the world stage and prepares for summits with South Korean President Moon Jae-in and US President Donald Trump, Beijing fears Pyongyang is moving out of its orbit and striking out on its own.
While China's economic pressure has been pivotal in bringing Kim to the negotiating table, Beijing worries Kim might now agree to a deal that brings his country closer to its old enemies and further from its traditional ally, which has seen it through famine and global isolation.
"There is even an extreme concern within the strategic Chinese community that maybe the US will accept a nuclear capable North Korea as its ally, or at least a friendly country," said Tong Zhao, a nuclear policy expert at Tsinghua Carnegie Center for Global Policy in Beijing.
Those concerns have been exacerbated by strained ties between the China and the US as the Trump administration clashes with an emboldened Chinese President, Xi Jinping, over trade.
"Some of the concern is so extreme that it almost sounds like a conspiracy theory, but it reflects this deep, embedded Chinese suspicion about both the US and North Korea," he said.
For more than half a century the status quo worked just fine for China -- North Korea, a fraternal communist country, acted as a buffer separating it from the US forces in the South.
But Pyongyang's pursuit of its own nuclear weapons, which intensified under Kim Jong Un, changed that balance -- raising the specter of a regional arms race on China's doorstep, the risk of preemptive military action by the US and its allies, or an unintended outbreak of conflict.
This was something Beijing could not tolerate.
"China has always wanted to maintain a normal and stable relationship with North Korea. China has no disagreements with North Korea in any issue area except nuclear," said Zhao.
"China had to respond strongly to North Korea's acceleration of nuclear development -- China had to join the rest of the international community to impose sanctions that really impacted North Korea."
Kim also purged several key officials with close ties to Beijing, including his uncle Jang Song Thaek, angering its chief patron.
Relations suffered and the two traditional allies ended up barely on talking terms.
The rapid overtures by Kim earlier this year proposing talks with South Korea and with the US, while not unwelcome, caught Beijing off guard.
China acted quickly to reassert itself. In late March, Kim turned up in Beijing, his first foreign trip since taking power seven years ago.
The optics were unmistakable. Kim showed deference to China by traveling to the capital to brief his huge neighbor and ally and to seek its advice and blessing. President Xi made a show of grandly feting Kim, in the process reminding the world that China is on North Korea's side and remains a key diplomatic player on the Korean Peninsula.
"China doesn't like the current picture of the two Koreas and the US center stage without China. Beijing's main concern is any chance that it can't exercise its influence and serve its interests in the region," said Duyeon Kim, senior fellow at the Korean Peninsula Future Forum in Seoul.
Will maximum pressure continue?
Last year, China signed onto the toughest ever UN sanctions because it wanted to bring Pyongyang to talks but it has no interest in regime change in North Korea.
Beijing fears both an economic and political collapse that would send refugees fleeing across the border and could lead to the potential reunification of the North with the US-allied South.
As such, when North Korea proclaimed last week it would no longer be testing nuclear weapons or missiles, some in China saw this as a chance to relax the sanctions that some analysts believe have forced the rapprochement.
Shortly after that announcement, an op-ed in the strident, state-owned Global Times stated this point of view explicitly: "If Washington still wants to coerce Pyongyang to abandon nuclear weapons with maximum pressure, it will be dangerous, and neither China nor South Korea will agree to such an approach. It will probably herald a return to even more intense turmoil."
"The international community should encourage North Korea by lifting some sanctions and resuming certain exchanges, showing North Korea the huge benefits its return to the international fold will bring, and the significance abandoning its nuclear weapons will have on its security. "
There is every indication that this opinion is shared by the Chinese government.
"We believe all resolutions should be implemented in their entirety and the DPRK sanctions and resolutions not only include the sanctions but also the measures that will encourage denuclearization and promoting peace and stability on the peninsula," said Foreign Ministry spokesman Lu Kang this week.
The White House insists that the US will not negotiate any concessions until Pyongyang takes concrete steps towards dismantling its nuclear and missile program and its maximum pressure campaign will continue.
Ultimately, Xi does not want the US to be in the driving seat in a region where China increasingly calls the shots, and at a time when relations between Beijing and Washington are at a low.
Kim, the analyst, says Beijing will exploit any opportunity to undermine US credibility and influence the results of the two summits.
"If Beijing isn't happy with the outcome of the two summits, it could easily derail American efforts to denuclearize North Korea by lifting sanctions against the North, providing a safe haven for its illicit activities, and refusing to implement existing and future UN sanctions," she says.