HONG KONG — The immediate causes of the recent diplomatic breakthrough on the Korean Peninsula are well known: stronger international sanctions against North Korea, approved by even China and Russia, and President Trump’s bellicose response to the recent intensification of nuclear and missile tests under Kim Jong-un, North Korea’s leader since 2011.

But a more fundamental driver is being overlooked: China’s growing ambition to dominate East Asia. Mr. Kim’s apparent move to reconcile with his South Korean counterpart, President Moon Jae-in, is above all a gambit to get closer to America to keep China in check. He hopes to reduce North Korea’s overarching economic dependence on China and curb Beijing’s aspirations to control the future of the Korean Peninsula. After another surprise meeting between Mr. Kim and President Xi Jinping of China on Tuesday, the second in two months, the Trump administration announced on Wednesday that North Korea would release three American prisoners.

The regime’s survival and security have long been the Kim family’s top priority, with political independence not far behind; those are the prime reasons it has sought to develop North Korea’s nuclear weapons and long-range missile capability. That purpose has also been served by political purges, notably the killing in late 2013 of Mr. Kim’s uncle Jang Song-thaek, who was suspected of entertaining especially close relations with China, and in early 2017 of Mr. Kim’s half brother Kim Jong-nam, another Beijing protégé and once an heir apparent to Kim Jong-il, the country’s previous leader and Mr. Kim’s father.

Now that these pressing existential objectives seem to have been satisfied, economic development has become the crux of the regime’s long-term stability. It is no coincidence, for example, that last month the Workers’ Party of Korea abruptly decided to abandon its well-established policy of byungjin — the simultaneous advancement of the country’s military, particularly its nuclear program, and its economy — to refocus entirely on economic development.