DISGUISED in the uniforms of paramilitary police, gunmen checked the ID cards of the passengers of a Karachi-bound bus before slaughtering all those they considered not native to the western Pakistani province of Balochistan. A total of 22 Pashtuns were killed in the attack on an isolated road south of Balochistan’s capital, Quetta, on May 29th. Responsibility was claimed by the United Baloch Army, part of a tangle of separatist groups in the province.

The insurgency began a decade ago and is now the most violent and long-lasting of five rebellions that have broken out in Pakistan’s largest, most thinly populated and least developed province since the country’s independence from Britain in 1947. That makes Balochistan, one of the most troubled areas of Pakistan, a surprising location for what officials hope will become one of the world’s great trade routes, linking the deepwater port of Gwadar with the city of Kashgar, a trading hub in the western Chinese region of Xinjiang (see map). During a visit to Pakistan in April by China’s president, Xi Jinping, it was announced that China would invest $46 billion by 2030 in new roads, the upgrade of existing ones, power plants, pipelines and other projects to fulfil this dream—far more than America has invested in Pakistan in recent years.

China hopes that the China-Pakistan Economic Corridor (CPEC), as it is called, will enable oil and gas from the Persian Gulf to be piped through Balochistan, over the Hindu Kush mountains, and into Xinjiang. Chinese goods would have a much shorter route to world markets than the one through the Malacca Strait, which China frets is at the mercy of America’s navy.

China has pledged investment before in Pakistan that has not proved as large as promised. This time, Pakistani officials say that nothing, not even the treacherous topography of the border with China, will be allowed to get in the way. Recently a tunnel was completed around a lake 14 miles (23km) long that had submerged a section of the Karakoram Highway traversing the two countries’ border. The lake was created by a landslide in 2010.

But Balochistan’s turmoil is a good reason to doubt that Chinese investment will flow in quite the abundance suggested during Mr Xi’s trip, at least not into that province. Despite evidence to the contrary—an attack by Balochs in January on the electricity grid knocked out power in 80% of the country, including the capital—officials dismiss it as a low-level insurgency run by infighting separatists whose ethnic group barely forms a majority in Balochistan. All will be fine, they say; thanks, not least, to the help of a 10,000-strong force that Pakistan promised Mr Xi would be set up to protect Chinese workers.

China, however, still worries. There have been occasional kidnappings and killings of Chinese workers in Pakistan in recent years, including in Balochistan. In March Baloch separatists attacked tankers carrying fuel to a Chinese company working on a mining project. Gwadar port, which was recently put under the management of a Chinese state-owned company, is a particular target. Militants do not want to see it developed unless Balochistan becomes independent. They fear that if it were ever to become a thriving port, then outsiders would flood in. That could weigh the province’s demographic balance even further against Balochs.

Pakistan’s previous attempts at pacifying Balochistan do not inspire confidence. Gung-ho generals intent on brutally suppressing the insurgency have succeeded only in intensifying it. The army rejects such charges: security in the province, it says, is in the hands of civilian-controlled police and the paramilitary Frontier Corps. These forces, however, have close ties with the army and intelligence services.