When North Korea's founder Kim Il-Sung visited Beijing to sign a mutual defence pact with China in 1961, he was comforted by the military protection promised by his fellow communist neighbours.
But half a century and a few North Korean nuclear tests later, the agreement is beginning to look like a musty Cold War relic that China would rather forget.
Despite their alliance in the 1950 - 1953 Korean War, analysts question whether Beijing would now rush to Pyongyang's defence in a military confrontation with the United States and South Korea.
Chinese leader Mao Zedong once described the neighbours as being as "close as lips and teeth."
For his part, Kim told Mao that signing the Treaty of Friendship, Cooperation and Mutual Assistance "raised our confidence, and we feel safeguarded", according to a memoir of Mao's diplomatic activities published in 2003.
But the two leaders are long dead and China, which is now the world's second largest economy and a pillar of the global order, appears less enthusiastic about protecting its treaty partner in a conflict that estimates suggest could cost hundreds of thousands of lives and lay waste to Seoul.
The current leaders of both nations, Chinese President Xi Jinping and North Korea's Kim Jong-Un, have never even met.
The treaty constitutes "a very important part" of Sino-North Korean ties, professor Maochun Miles Yu at the United States Naval Academy told AFP.
But, he added, it is by no means clear what China is actually prepared to do for the North if push comes to shove: "It's a mystery."
- Atomic threat -
North Korea's regular missile launches and the prospect of a possible sixth nuclear test are putting Beijing in a tight spot as it already faces US pressure to slash economic ties to punish Pyongyang.
"It's hard to say how China would assist North Korea militarily in case of war, since North Korea is developing nuclear weapons, an act that might have already breached the treaty between the two nations," retired Chinese naval colonel Li Jie told the daily South China Morning Post last month.
China's nationalistic Global Times newspaper invoked two scenarios, including one in which Beijing would refrain from defending Pyongyang.
"If North Korea continues to carry out severe missile tests, and the United States launches a surgical attack on its facilities, Beijing should impose a diplomatic boycott, but there is no necessity of military intervention," said an op-ed published in the Global Times last month.
But the daily said China "should immediately carry out necessary military intervention" if the US and South Korea launch a ground invasion in the North to overthrow the regime.
While Beijing's commitment to the pact is in question, it remains sensitive to US military movements in the region.
On Tuesday, China demanded that Washington "immediately" suspend the deployment of a missile shield in South Korea hours after officials announced that it was operational.
"We will firmly take necessary measures to uphold our interests," foreign ministry spokesman Geng Shuang said.
- 'Complicated situation' -
The military pact automatically renews every 20 years, most recently in 2001, meaning it is now valid through 2021.
Last year, Xi and Kim exchanged messages to mark the treaty's 55th anniversary.
The Chinese president wrote that both sides have cooperated in the "spirit" of the treaty and that their "friendship serves as a precious wealth."
The treaty, Kim responded, "has become a firm legal foundation for constantly consolidating the friendly and cooperative relations that were forged in the bloody struggle for independence against imperialism and for socialism."
But in 2013, a Chinese defence ministry spokesman said it would be "unprofessional" to answer a hypothetical question about whether Beijing would militarily back Pyongyang in an attack.
Asked on Tuesday whether China was still committed to the pact's terms, foreign ministry spokesman Geng Shuang said: "The principle of (the treaty) is to promote China–DPRK friendly cooperation in various fields and uphold regional security."
He added: "The current situation on the peninsula is highly complicated, delicate and tense. We urge all sides to stay calm and abstain from any action that may aggravate tensions."
Young-June Chung, an associate professor at Shanghai's Tongji University, said the pact's "diplomatic status is a little bit obscure" as neither side has acknowledged that it is "invalid."
But he added that Beijing "would not sacrifice its relations with the US and South Korea for North Korea."