(CNN)A rocky, uninhabited island chain in the East China Sea has soured relations between Beijing and Tokyo for decades.
Tensions have flashed numerous times over the Senkaku Islands, which China calls Diaoyu Islands, including face-offs between Japanese and Chinese warplanes and ships.
Headlines about Asian tensions have been dominated by China’s activities in the South China Sea, but analysts say the dispute further north is approaching a slow boil.
This week, three Chinese Coast Guard ships sailed near the islands — after new US Defense Secretary James Mattis reaffirmed US commitment to defending Japan and its disputed islands.
Here’s what’s at stake:
Who claims what?
China says its claim to the islands extend back to 1400s, when it was used as a staging point for Chinese fisherman.
However, Japan says it saw no trace of Chinese control of the islands in an 1885 survey, so it formally recognized them as Japanese sovereign territory in 1895.
A group of settlers manufactured dried fish and collected feathers, with the islands having more than 200 inhabitants at one point, according to Japan’s Foreign Ministry.
Japan then sold the islands in 1932 to descendants of the original settlers, but the factory failed around 1940 and the islands were eventually deserted. The Japanese surrender at the end of World War II in 1945 only served to cloud the issue further.
The islands were administered by the US occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.
Since then, the US has stated ownership of the Senkaku/Diaoyu Islands is a matter between China and Japan, but, under treaty obligations, the US says it would defend the islands because they are administered by Japan.
Taiwan, which Beijing regards as a breakaway province, also claims ownership of the chain.
The most recent round of tensions was lit in 2012, when Japan nationalized the islands to ward off a planned sale to Tokyo’s then-governor, a hardline nationalist apparently hoping to develop the islands.
The plan sparked massive anti-Japanese protests across China.
Demonstrations turned violent as protesters hurled bottles at the Japanese Embassy in Beijing, overturned Japanese cars and ransacked Japanese stores and restaurants.
In 2013, China declared a formal Air Defense Identification Zone covering airspace over the islands and overlapping with airspace claimed by Japan. The ADIZ declaration required airlines flying over the waters to first notify China.
The move prompted an outcry from Japan and the United States, but a US report last year suggested that it was not being fully enforced.
Why are the islands considered valuable?
The discovery of potential oil, natural gas and methane hydrate deposits in the area has added impetus to the dispute, says Carl Schuster, a professor at Hawaii Pacific University and former director of operations at the US Pacific Command’s Joint Intelligence Center.
The islands are located in the Shirakaba/Chunxiao gas field. The two sides did agree in 2008 to jointly develop the resources, but Japan has since accused China of unilateral drilling.
According to a 2016 report by the Asia Maritime Transparency Initiative, China has at least 12 operational drilling rigs.
It added that Japanese defense officials fear that China might make military modifications to the rigs but concluded it was unclear whether China intends to use the rigs for civilian-military use.
The islands are also close to strategically important shipping lanes and fertile fishing grounds.
What are China’s ships doing?
Since 2012, Chinese Coast Guard vessels and fishing boats have begun to ply the waters in growing numbers and have been accused of bullying Japan’s fishermen over the last two years, says Schuster.
On one occasion in August last year, more than 200 Chinese fishing ships entered the waters around islands, accompanied by China Coast Guard vessels, according to the Asia Maritime Transparency Initiative.
“Their Coast Guard vessels are larger than Japan’s and, in some cases, better armed albeit with defensive weapons only,” Schuster says.
What’s more, he says, “a portion of China’s fishing fleet is a paramilitary organization called the People’s Maritime Militia. They have better communications gear than a standard fishing craft and their crews are armed and receive military training. “
He added that China views the patrols as no different to the Freedom of Navigation patrols that the US conducts regularly in the South China Sea.
The February 6 patrol, which went within 12 miles of the islands, was the fourth in 2017, according to Japan’s Coast Guard. Last year, there were 36 such incidents.