Their faces smeared in green and black, some with Stinger anti-aircraft missiles on their packs, the men of “Darkside"—the 3rd battalion of America’s 4th marine regiment—boarded a pair of Sea Stallion helicopters and clattered away into the nearby jungle. Their commanders followed in more choppers carrying ultralight vehicles and communications gear. Anything superfluous was left behind. No big screens for video links of the sort used in Iraq and Afghanistan: to avoid detection, the marines must make sure their communications blend into the background just as surely as their camouflage blends into the tropical greenery. The goal of the exercise: to disperse around an unnamed island, link up with friendly “green" allies and repel an amphibious invasion by “red" forces.

Ignore the polite abstractions. The marines are training for a war with China, probably precipitated by an invasion of Taiwan. Their base in Okinawa, at the southern end of the Japanese archipelago, is just 600km (370 miles) from Taiwan. The two islands are part of what American military planners call the “first island chain": a series of archipelagoes and islands, big and small, that stretches from Japan to Malaysia, impeding naval passage from China to the Pacific. Whether by harrying Chinese ships from a distance or—much less likely—by deploying to Taiwan to help repel a Chinese landing, the marines will be early participants in any conflict.