Beijing is building an airstrip in the South China Sea and has chosen a week when American naval attention is pointed at Iran. Reuters reported on October 2 that satellite imagery shows Chinese work on Antelope Reef gathering pace.

Antelope Reef sits in the Paracel Islands. China holds the Paracels, and Vietnam and Taiwan both claim them.

A runway on a reef is not a decoration. It extends how long aircraft can stay over the water, and it gives the coast guard and the navy somewhere to land, refuel and rotate. China did this at Woody Island in the same chain, and the pattern is familiar to anyone who has watched Beijing turn a claim into a facility.

The timing comes with its own calendar. The Guardian reported on October 2 that a third US aircraft carrier is heading to the Middle East for the Iran war, as American midterm elections loom. Another outlet put the surge of US troops into the Gulf at 20,000. Carriers are finite. Every one assigned to the Gulf is one that is not in the western Pacific.

Beijing does not need to time its work to Washington's deployments. It only needs to keep working, and the war supplies the cover. The Philippines was the target of joint PLA naval and air drills around Scarborough Shoal days after a standoff. Taiwan's defence ministry counted 23 Chinese military aircraft and seven naval vessels around the island in a single 24-hour period this week. Different waters, same tempo.

The Paracels sit north of the Spratlys, close to Vietnam's coast and within reach of the sea lanes that run to Taiwan and Japan. A hardened runway there is a fixed asset that Beijing will not give back, and no diplomatic communique covers it.

If the read is right, China is using the Iran war to bank South China Sea facilities that outlast the war, and the price will be set by Vietnam and the Philippines, who have the least ability to pay it.