In the chilly dusk on a recent Saturday, nine people were creeping through the California hills. Their faces were painted in shades of green, yellow, brown and black, so that they blended into their surroundings. Moving uphill through thickets of trees, they tried to be silent, as if not to draw the attention of some unseen enemy. They were conscious of every breath, every dry leaf that crunched underfoot, every snapped twig.

These people were not military personnel. They were just civilians — biotech workers, a masseuse, an entrepreneur — who had decided to spend a weekend preparing themselves for a war, societal collapse or some other calamity.

A booming voice broke the silence: “Camo! Five, four, three, two, one!”

The person giving the order was Jessie Krebs, a wilderness expert who has trained hundreds of U.S. Air Force officers in how to stay alive behind enemy lines through an intensive course called Survival, Evasion, Resistance and Escape, or SERE.

“Remember,” Ms. Krebs, 51, called out. “Your mission is to hide from me. You want to put objects between me and you. If you’re in the open, you don’t want to look human. You want to evade detection from the enemy.”