An old saying warns about Greeks bearing gifts, but it might fit the Chinese better. In the 1400s Zheng He, a Muslim slave who became the Ming empire’s admiral, led seven voyages south and west. He offered treasure to every leader he met—but only if they acknowledged the emperor, joining a world order centred on Beijing.

Chinese leaders today are following in Zheng’s wake. The “road” half of its Belt and Road Initiative (bri)—a global infrastructure-building scheme—is a maritime one of seaports and shipping channels. Xi Jinping, China’s president, has said it will create a new model of “win-win co-operation”. Some critics suspect nefarious motives, such as yoking poor countries to China by giving them unrepayable loans.

The BRI has evolved site by site and Chinese officials have not made their intentions clear. However, the locations of the 22 maritime-road projects that we have identified as under way show how it is most likely to aid China. They suggest it will be more useful for protecting existing trade routes than expanding Chinese influence.

To measure the maritime road’s impact, we tested three benefits it could offer China. If the road were a resource grab, its projects should cluster in places that sell raw materials that China imports. If its aim were to boost trade, it should track the busiest routes used by Chinese shipping today, or where trade is likely to grow fastest. And if it were intended to secure current trade routes, its ports should sit near choke points—areas whose closure would force goods to travel circuitously—or in places that offer alternative routes.

We tested these explanations by using them to predict if countries host a BRI port. The results were conclusive. After holding other factors constant, there was no statistically significant link between having a bri port and exporting raw materials that China wants, or having high current or projected trade with it. In contrast, the “trade-protection benefit”—either the value of Chinese trade in a country’s waters multiplied by the extra distance goods would have to go if those routes were shut, or the amount of trade that would be diverted to a country if shipping were disrupted elsewhere—was a good predictor. Given two otherwise average countries, one with a high trade-protection benefit (like Libya) is 2.7 times likelier to host a bri port than another with an average benefit (like Liberia).

Owning or running a port does not guarantee perpetual access, but it does give China influence by enabling it to disrupt the host’s own shipping if it chooses. Many overland “belt” routes in the bri would also make Chinese trade more resilient. For example, if the Strait of Malacca were closed, China could switch to bri ports it wants to build in Myanmar, and finish the trip on planned bri rail lines.

China’s military footprint also shows a focus on guarding trade routes. Its only base abroad is at Djibouti’s Bab al-Mandab Strait—the waterway whose closure would hurt China more than anywhere else.