Moving closer:Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping in Beijing in November 2014.APKim Kyung-hoon  

It was the smile that did it.

When Shinzo Abe, Japan’s Prime Minister, met with President Xi Jinping of China on the sidelines of a regional summit meeting in Vietnam over the weekend, the pair shook hands and posed for a photo. Mr. Xi, who had looked more dour in previous meetings, flashed a grin.

It was a sign, the Japanese news media suggested, of warming relations between the two long-time adversaries.

With President Donald Trump creating unease among allies about the role the U.S. will play in the region, Japan and China are inching toward a possible rapprochement as they recognise the shifting dynamics around the Pacific Rim.

A realignment?

But with the two Asian powers long divided by disputes over history and territory, as well as testiness over influence in the region, it will take more than a few handshakes — or a smile — to cement a genuine realignment.

In gesturing toward a new friendliness, Japan is motivated in part by the recognition that China is supplanting the U.S. as the leader of free trade in the region.

Having watched Mr. Trump heap praise on Mr. Xi in Beijing last week, Japan is also propelled by fear that the U.S. may develop a closer relationship with China that would exclude Japan. And as China seeks to consolidate its power, it realises it may have more success exerting its authority in the region with Japan as a partner rather than a pure rival.

At the same time, Mr. Trump’s visit showed China that the U.S. is unlikely to get in its way, allowing a more confident Mr. Xi to be more generous toward Japan.

Real shock

“What Trump represented was a real shock to the system of allies and the world,” said Nick Bisley, professor of international relations at La Trobe University in Melbourne, Australia.

The threat from North Korea is also naturally drawing China and Japan together, although Mr. Abe has so far hewed closely to Mr. Trump’s approach of calling for more pressure and sanctions.

Mr. Abe appears keenly aware of Mr. Trump’s erratic swings in opinions and loyalties. Japan is naturally wondering if the U.S. may make some kind of deal with China that could put Japan at a disadvantage, experts said.

“Standing behind that is kind of what you always find in Japan — that underlying fear of abandonment,” said Daniel C. Sneider, a lecturer in East Asian studies at Stanford University. As Mr. Abe and Mr. Xi take the first steps toward a better relationship, many obstacles remain.

Neither country has given any ground in a territorial dispute over a set of islands in the East China Sea, known in Japan as the Senkaku and in China as the Diaoyu. China also still regularly objects to what it perceives as any sign that Japan is returning to its militaristic past and carefully watches the debate in Japan about how much to develop the military’s capabilities.

Trade pact

Japan has also actively sought to counter China’s economic rise by developing relationships with other countries in the region. On the same day Mr. Abe met Mr. Xi in Danang, Japan led a group of 11 countries in announcing the restart of negotiations for a sweeping trade agreement to create an economic bloc that would exclude China.

In both countries, the public views the other with suspicion. “I don’t think I see how they can make drastic improvements, given the public opinion base,” said Ezra Vogel, a professor emeritus of social sciences at Harvard who is working on a book about Sino-Japan relations.

Still, the tentative outreach is happening as both Mr. Abe and Mr. Xi have recently shored up their domestic power, in Japan through a parliamentary election in October and in China as a result of last month’s Communist Party congress.NY TIMES