DAKAR, Senegal — When United States forces set out on what was ultimately a deadly joint patrol with Nigerien soldiers this month, they were entering terrain crisscrossed by criminal elements and terrorist organizations in a dangerous, yet often forgotten, corner of the world.

The area in Niger, near the border with Mali, is known to host groups loyal to both Al Qaeda and the Islamic State and has long been used as a smuggling route for arms, drugs and people. Just over a week ago, gunmen killed 13 Nigerien soldiers in an attack on their base, not far from where the joint patrol was ambushed, the latest in dozens of assaults unleashed in the past two years.

The chief suspect in the Oct. 4 ambush is a group affiliated with the Islamic State operating in a desolate area that has long suffered from poverty and government inattention.

“They are presenting an alternative to a state that villagers associate with corruption and neglect,” said Corinne Dufka, a regional director for Human Rights Watch who has studied the area for years.