Recent joint operations by the Cameroonian and Nigerian militaries have captured and killed numerous fighters and seized suicide belts, weapons and equipment for making mines. Officials hope to squeeze the fighters from both sides of the border so they have nowhere left to run.

But the multinational military force — which includes Chad, Niger and the American military in an advisory role — sometimes has trouble securing territory once it is cleared.

In Cameroon, soldiers drove Boko Haram fighters from the border town of Kerawa in October, but it has since been assaulted at least six times, with militants beheading a young man in one attack. Early last month, a suicide bomber entered Kerawa and blew himself up behind a house, killing a tailor. According to a State Department report on the episode, the bomber had been looking for a large group of people to kill, but after all the attacks in the town, the streets were empty.

The mass displacement caused by Boko Haram — and by the sometimes indiscriminate military campaign to defeat it — has left 1.4 million people in the region without adequate food supplies, the United Nations says. In Borno State in northeastern Nigeria, where the situation is the most acute, humanitarian workers say that 50,000 people are one step away from famine. Along the Chadian border, farmers typically trade their pepper crops for imported cereals and grains. But the pepper fields have been abandoned and there is little left to trade.

“We’re looking at a large-scale crisis in very remote areas,” said Denise Brown, the West and Central Africa regional director for the World Food Programme. “This is not a today problem. This is a tomorrow problem and a next year problem unless it’s contained, and I don’t see it coming to a halt soon.”

In the Far North of Cameroon, this time of year is a moonscape of bone-dry river beds and clouds of dust so thick they look like misty fog. The region is moving into the so-called lean season, the in-between months when the fruits of the previous harvest are being depleted and next year’s crop is not yet ready.