The Dongwha Duty Free Shop in the Gwanghwamun district in downtown Seoul is regularly thronged with visitors from China, some of the 4 million who now visit South Korea annually, making it the top foreign tourist destination for Chinese travellers.
At a nearby information centre, officials from the Korea Tourism Organisation answer questions in Chinese, now so widely used that it has replaced English as the second language at Incheon International Airport.
The relationship between the two countries is becoming so close that it seemed only natural in early July for Chinese President Xi Jinping to tell his South Korean counterpart, Park Geun-hye, that his official trip to Seoul felt like "a visit of relatives."
But Xi and Park are not the only Northeast Asian leaders balancing on the diplomatic high wire. Almost at the same time Xi was chatting with Park in Seoul, Japanese Prime Minister Shinzo Abe was announcing the relaxation of a raft of unilateral sanctions imposed on North Korea in 2006.
Abe's move was in response to apparent progress on securing the release of dozens of Japanese citizens allegedly abducted by North Korea in the 1970s and 1980s. But it was also calculated to strengthen Japan's bargaining power in Northeast Asia by opening a direct diplomatic line from Tokyo to Pyongyang.
Japan has long been part of international efforts to counter North Korea's nuclear program, not least as a member of the so-called six-party talks, which also involve China, the US, South Korea and Russia. It was also a co-sponsor of a recent United Nations resolution on North Korea's widely condemned human rights record.
The Japan-North Korea detente caught many by surprise. But by rewarding Pyongyang's expressed willingness to investigate the abductions, Tokyo has created new strategic possibilities, depending on the outcome of the inquiry, which is expected in August.
One possibility is a summit between Abe and Kim Jong Un, the North Korean leader, which the Japanese prime minister is believed to want to hold before the end of the year. Abe has already been to Pyongyang once, as chief negotiator on the abductee issue during then-Prime Minister Junichiro Koizumi's visit in 2002. If he makes a second trip, the region would have two unexpected relationships: Japan with North Korea, a close ally of China; and China with South Korea, which relies on the US, also a close ally of Tokyo, to defend it against any attack by North Korea.
This fast-changing strategic landscape reflects rapidly developing views in several capitals about the future of the long-term relationships tying Japan and South Korea to the US, and North Korea to China, that have maintained a balance of power in the region since the end of the Korean War in 1953.
For a start, China and South Korea share a rising antagonism toward Japan, which both accuse of glossing over the crimes of the Imperial Japanese Army before and during World War II. The two countries also have active maritime territorial disputes with Japan, while calm has prevailed in both Beijing and Seoul in relation to their own relatively minor maritime dispute.
As the hordes of Chinese tourists in Seoul demonstrate, the two countries also share an increasingly important economic relationship. China is South Korea's largest trading partner – worth a total of $203 billion in two-way trade in 2012, more than South Korea's combined trade with the US and the European Union. Reinforcing bilateral relations, nearly 1 million South Koreans were working in China last year.
Meanwhile, China and North Korea are drifting apart. China appeared less than happy with Pyongyang's most recent nuclear tests. The North Korean leader's decision last December to execute his uncle, Jang Song Thaek, who was Beijing's most trusted middleman, has added to tensions. And Beijing's endorsement of UN financial sanctions against North Korea last year may have hardened Pyongyang's attitude. Notably, the North Korean leader has not yet visited Beijing.
For Pyongyang, improved ties with Japan would bring economic benefits and much-needed humanitarian aid, and perhaps assist its efforts to normalize relations with the US through a peace treaty, though that remains little more than an aspiration.
Pyongyang may also hope a cozier relationship with Japan will reinvigorate the stalled six-party talks, calculating that the international community still wants to engage North Korea to prevent any sudden collapse of the regime, which could have serious consequences for both China and South Korea.
For all four governments, the stakes are high. China stands to gain much from a closer relationship with South Korea, both economically and strategically, especially if it can drive a wedge between the two main US allies in the region. Seoul seeks to entrench its increasingly valuable economic relationship with China and, perhaps, to discourage Beijing from underwriting any further aggression by North Korea.
Tokyo is pursuing twin goals: a conclusion to the emotionally wrenching abduction saga and a larger role, commensurate with its economic clout, in Northeast Asian diplomacy.
These emerging relationships may presage a more permanent expansion of cordial ties that could ease tensions considerably over time. But they are also marriages of convenience that could fall apart as quickly they have been established.
(First appeared in Nikkei Asian Weekly, www. asia.nikkei.com).