One by one, the American soldiers slid down a muddy hillside to a river deep in the Hawaiian jungle. With guns on top of rucksacks, they kicked their way across, wobbling in the current, trying to stay quiet.

It was a sluggish advance stinking of sweat and silt — reminiscent of Vietnam, and similar to what they might face in a potential fight with China almost anywhere in the Pacific.

“It’s incumbent on us to become resident professionals,” said Col. Christopher D. Johnson, who traversed the river arm in arm with a junior officer. “Firemen don’t figure out how to work a fire engine at the fire, right?”

Military strategists like to say the jungle is neutral, helping neither friend nor foe, but for most of the 79 students last month at the U.S. Army’s only jungle school, “the J” was just plain new. It was nothing like home or deployments to Iraq and Afghanistan, which explained why they were there: To deter China and work effectively with partners in the region, American ground forces need more jungle expertise.