Over the past decade, the Sahel, a vast semiarid region of western and north-central Africa, has become a tangle of transnational terrorist groups, including the Islamic State in the Greater Sahara, Boko Haram and Jama’at Nusrat al-Islam wal-Muslimin. France’s eight-year military campaign intended to stabilize the region, named Operation Barkhane, ended in failure in the fall of 2022, leaving a security vacuum that was quickly filled by jihadists and Wagner mercenaries.

Contrary to the postcoup narrative, Niger — while a democracy — was hardly an oasis of stability: The Global Terrorism Index has documented a steady increase in terrorism-related deaths in the country in recent years. But the nation’s successive elected governments were at least willing to cooperate with Washington, allowing the U.S. military to conduct regional counterterrorism activities. The United States has two military bases in Niger with roughly 1,100 troops between them, cooperated with government officials and operated a security cooperation assistance program for Nigerien troops fighting Al Qaeda and Islamic State militants in the Sahel.

Now military exercises between America and Niger have been suspended. Washington has stopped short of calling the crisis a coup — a move that would require the United States to halt security and economic assistance. American diplomats and West African officials are trying to negotiate a return to power for Mr. Bazoum. If that effort fails, and Washington loses access to the drone base it runs there and other intelligence and surveillance activities in the area, its grasp of what insurgent groups are up to in the Sahel will be severely curtailed.

Wagner will be ready. Its forces are already deployed in Mali and Libya, both of which border Niger, as well as in the Central African Republic and Sudan. Since first sending troops to Africa in 2017, the group has embedded itself in these fragile states and siphoned valuable resources, a quid pro quo that offers military muscle in exchange for mining contracts that allow Wagner subsidiaries to extract gold, diamonds and other commodities that pad Russia’s coffers. Their operations have frequently resulted in the deaths of civilians, with credible accusations of sexual violence, torture and extrajudicial killings. The arrangements boil down to simple supply and demand: African putschists need the security that Wagner can provide, and the Kremlin needs the funding stream to soften the blow from biting Western sanctions.

That system did not collapse after Mr. Prigozhin’s failed coup. Within days of Wagner’s aborted advance on Moscow, Russia’s foreign minister, Sergei Lavrov, said that Wagner’s African footprint would remain. Late last month, Mr. Prigozhin reportedly surfaced from exile in St. Petersburg, where he posed for photographs with African leaders who were in town for a summit. Mr. Putin also attended.