The United States has about 800 service members in Niger, yet the scale of its military operations there surprised even two high-ranking senators: Chuck Schumer of New York, the Democratic leader, and Lindsey Graham, an influential South Carolina Republican, said in recent interviews on “Meet the Press” that they had not realized the extent of the military’s involvement in Niger.
“We don’t know exactly where we’re at in the world, militarily, and what we’re doing,” Mr. Graham said.
About 6,000 American troops are in Africa, most of them in Djibouti. About one-third of all troops are Special Forces or Special Operations forces, many working on advise-and-assist missions like the patrol in Niger.
Niger and other West African countries have a long history of working with American forces. Niger last hosted major military training exercises carried out by Special Operations forces in 2013 and will do so again early next year.
In Niger, several hundred American troops work at the drone base in the capital, Niamey, and about 200 others have arrived to work at a $50 million drone base under construction at Agadez.
Niger is host to some of the continent’s biggest problems. Aside from the terrorist groups operating on its western borders, Niger is fighting the Islamist militant group Boko Haram on its southeastern frontier. The country suffers from overpopulation, with one of the highest birthrates in the world, and ranks as one of the world’s poorest nations.
The lack of government attention to rural regions is among the factors thought to have helped spawn militant groups and, in recent years, to have helped the insurgents to recruit.
No group has yet claimed responsibility for the Oct. 4 attack, which occurred near the border village Tongo Tongo. But both the Americans and Nigeriens have said that the chief suspect is ISIS in the Islamic Sahel, an outfit that has pledged allegiance to the Islamic State. They suspect a village leader stalled the convoy when soldiers entered, allowing militants time to assemble and prepare their ambush, which occurred not far from Tongo Tongo.
French security officials say the ISIS branch has 40 to 60 core members, but it is often joined by sympathetic villagers and has temporary alliances with other local groups. The officials, who spoke on the condition of anonymity because of their diplomatic protocols, said that French and Nigerien patrols have in recent months engaged in minor skirmishes with militants in the border region where the Americans were killed.
Militants, including the ISIS fighters, have proved to be adept at staging hit-and-run attacks, with machine-gun-toting fighters on motorcycles and more heavily armed insurgents in pickup trucks launching quick, harassing strikes and then melting into the desert.
The ISIS branch and its leader, Adnan Abu Walid al-Sahraoui, sought the recognition of the Islamic State in 2015, according to the United States Defense Department. Its pledge was acknowledged more than a year later, in October 2016, through a media channel tied to the Islamic State.
Around that time Mr. Sahraoui’s group carried out several attacks in the region, including operations against militias loyal to French and United Nations peacekeeping troops deployed in northern Mali.
ISIS in the Islamic Sahel — Sahel refers to a band of desert that stretches from the Atlantic Ocean to the Red Sea — has also attacked a customs outpost in Burkina Faso, near the borders with Mali and Niger, and tried a jailbreak in Niger at a building housing suspects from Al Qaeda groups and Boko Haram.
The group is smaller than other organizations in the region, and many security experts suspect that the militants did not realize they were attacking American forces that could easily overpower them in planned operations.
There are no indications that the Islamic State has directed any attacks in Niger or offered any guidance. But analysts said that was not surprising given Mr. Sahraoui’s knowledge of the area.
“These guys are experienced fighters,” said Andrew Lebovich, a visiting fellow at the European Council on Foreign Relations and a researcher on security and political issues in the region. “They have actively been conducting operations for several years.”
Operating in a vortex of ethnic tension, Mr. Sahraoui has an extensive background in the Islamist insurgent groups active there since the 1990s.
In his late 30s or early 40s, Mr. Sahraoui is believed to have spent some of his childhood in refugee camps in Algeria during the Western Sahara War and later joined the fight to create an independent state in Western Sahara. Eventually he joined Islamist groups that made their way to Mali.
He once referred to himself as “emir” of a group with ties to Al Qaeda that in 2012 took control of territory in Mali, instituting its own rule until French troops ousted it in 2013. He has occupied leadership roles alongside another of the region’s most-wanted terrorists, Mokhtar Belmokhtar.
Infighting in the region’s offshoots of Al Qaeda in recent years has led to mergers, the formation of splinter outfits and eventually the alignment of Mr. Sahraoui’s cluster with the Islamic State.
While it does not appear that groups loyal to Al Qaeda are cooperating with ISIS in the Islamic Sahel, they have not fought in the recent past, leading analysts to suspect they are communicating.
“The fact that they both operate in the same regions suggests at least some manner of contact between them,” Mr. Lebovich said.
Other groups loyal to Al Qaeda are thought to be responsible for a string of attacks in the past two years on hotels, cafes and resorts frequented by Westerners in Mali, Burkina Faso and Ivory Coast. This month, the United States Embassy in Senegal warned Americans to avoid seaside hotels and businesses in Dakar.
The tactics of insurgent groups in the region have shifted in recent years, according to a report from the International Crisis Group. Instead of trying to seize control of major towns, they are now using bases in the countryside to launch strikes. In some areas, the tactic has forced national security forces and local authorities to retreat, leaving large rural areas in the control of insurgents.
The terrorist organizations in the area compete for recruits among the booming population of young people in a region that has enormous unemployment. Niger has joined regional coalitions, which have sent military units to help in its fight against terrorism.
In southeastern Niger, the Chadian military has helped patrol areas as part of a task force of nations that face regular attacks from Boko Haram, which also has a faction that has pledged allegiance to the Islamic State. This month, Chad pulled its troops out of Niger, a move some analysts worry will have violent consequences for hundreds of thousands of people fleeing an insurgent battle that the national military has been ill-equipped to fight.