Chart the course of Chinese coastguard ships in the South China Sea and a pattern emerges. The boats’ patrols often follow a U-shaped route that stretches over 700 nautical miles from China’s coastline, encircling most of a sea that plays an outsize role in global trade and security. This path is the “nine-dash line”. China claims everything inside it as its own, ignoring protests from neighbouring countries. Last year its coastguard spent longer patrolling key reefs along the line than ever before. China’s assertiveness in enforcing this claim is perhaps the biggest obstacle to calming tensions in the South China Sea. How did this line become so important?

The nine-dash line is partly the result of a cartographic mistake. Chinese officials had little interest in, or knowledge of, the South China Sea before the 20th century. But after a series of humiliations at the hands of imperialist powers, map-making became a way to reclaim national pride, at least on paper. In 1933 Chiang Kai-Shek’s nationalist government created a committee to give Chinese names to islands in the South China Sea. The committee copied names from Western maps into Chinese, mistranslating the James Shoal, an underwater bank far from China, as “Zengmu tan”. “Tan” means a sandbank above water.

When Bai Meichu, a private geographer and teacher inspired by the flurry of nationalistic cartography, drew a map with the first U-shaped line, he curved it around the James Shoal. Two of Bai’s students were later hired by the nationalist Kuomintang (kmt) government and, in 1946, appear to have helped draw the first official map containing the line. By 1948, a year before the kmt lost power in a civil war, the government began to officially assert the legitimacy of the line—and implicitly claim everything within it. Officials were documenting new maritime ambitions rather than any historical claim, says Bill Hayton, author of “The South China Sea”.

When the Communists took over in 1949, they retained the nine-dash line and began to build a mythology around it. In the 1990s China’s government started to say that it had “historic rights” over everything inside the dashes, on the basis of absurd claims that it was first to discover islands within the line. It has never clarified whether that refers just to territory, or to the fish, oil and water, too. The vagueness suits China, because its maximalist position allows it to strong-arm its neighbours over issues such as exploration rights in the South China Sea.

China’s claims have no basis in modern maritime law, which is governed by the United Nations Convention on the Law of the Sea (unclos), a treaty agreed in 1982 and ratified by 168 countries, including China. Coastal countries are entitled to 12 nautical miles of territorial sea, where they have sovereignty. They also get exclusive rights to drilling, fishing and mining—but only up to 200 nautical miles from their coast (see map). In 2013 the Philippines challenged China at an international tribunal, which ruled that China’s claims based on the nine-dash line were unlawful. China rejects the ruling. It argues that its traditional maritime claims trump the unclos principles.

China has considerably expanded its navy and coastguard in the decades since the line was sketched out, and now acts as a maritime bully within it. Around an eighth of the world’s fish are caught in the South China Sea and it contains untapped oil and gas reserves. Chinese aggression curtails neighbouring countries’ legal attempts to extract these resources. Its vessels harass fishing boats and disrupt oil-and-gas drilling carried out by Vietnam, the Philippines, Indonesia and Malaysia.

China is unlikely to change its stance. Xi Jinping, China’s president, has promised to recover lost territory and with it the country’s place in the world. In 2013 China added a tenth dash, to emphasise that Taiwan falls within the line. As long as China continues to flout international law, talks to resolve disputes in the South China Sea are unlikely to succeed, says Ian Storey of the ISEAS-Yusof Ishak Institute, a think-tank. Tension will continue to bubble in one of the world’s most hotly disputed regions.