No More Coups in West Africa, Nigeria’s Leader Vowed. Niger Called His Bluff.

When President Bola Ahmed Tinubu of Nigeria took the helm of the West African regional bloc of countries last month, he thundered before a roomful of his presidential peers that he would show no tolerance for military coups in an area that had faced five in less than three years.

“We will not allow coup after coup,” he said, drawing a round of applause.

Two weeks later, mutinous generals took power in neighboring Niger, prompting Mr. Tinubu and the regional bloc, the Economic Community of West African States (known as ECOWAS) to draw a line in the sand: The mutineers in Niger had a week to relinquish power and release the president or face consequences, including military action.

Now, the deadline has passed, Niger’s president — Mohamed Bazoum — is still held hostage in his residence and Mr. Tinubu is facing a backlash in his own country.

Senators, religious leaders and civil society organizations in northern Nigeria oppose a war with a neighbor that they say would further destabilize both countries, whose militaries were already spread thin fighting off Islamist militants. Nigerian security forces are also combating kidnappers, extortion rings and oil thieves.

Northern Nigeria and Niger share not just a nearly 1,000-mile border, but also ethnic ties, language and a livelihood from active trade — leading even Mr. Tinubu’s military leaders to strike a cautious tone.

“Niger and Nigeria are going to be next to each other forever,” Gen. Christopher Gwabin Musa, Nigeria’s chief of defense staff, the most senior uniformed military adviser to the president and minister of defense, said in an interview. He likened a conflict with Niger’s coup leaders to “fighting your brother.”

On Thursday, Mr. Tinubu is set to gather his peers from other countries in the West African bloc for a summit in Nigeria’s capital, Abuja, where all options are on the table, according to a presidential spokesman. The outcome could affect the future of Niger and the stability of a region that has become the world’s epicenter of Islamist insurgency.

The crisis in Niger presents an immediate test for Mr. Tinubu, the recently elected president of Africa’s largest economy, who has vowed to reposition Nigeria as a geopolitical leader in the region. His decisive moves to steady the nation’s economy earned plaudits from investors, but also anger from the public over a resulting spike in food and fuel prices. His opponents have gone to court, alleging that his victory in the February election was fraudulent.

The Niger coup also has resurfaced the limits of the Economic Community of West African States, an entity founded in 1975 that has faced persistent criticism for its inability to stop military takeovers and accelerate regional economic integration. Three member countries have already been suspended because of coups in recent years, and Niger could be the fourth. Mediation efforts with Niger’s junta have so far stalled.

J. Peter Pham, a former U.S. special envoy to the Sahel, said of Mr. Tinubu, “Perhaps because he is not a military man, he was not entirely cognizant of the political and operational complexities of what ECOWAS threatened, challenges which are compounded when one starts thinking about a joint endeavor.” He added: “Thus, ECOWAS may find its credibility tarnished, rather than enhanced.”

Mr. Tinubu is a 71-year-old veteran of Nigerian politics who was briefly jailed by military rulers in the 1990s and forced into exile. Nigeria, like Niger, has experienced five coups since independence in 1960.

“It is unacceptable to him and his fellow heads of state that at this modern age of continental history, we’re still talking about forceful change of power at the barrel of a gun,” Ajuri Ngelale, Mr. Tinubu’s spokesman, said in an interview.

After generals in Niger removed Mr. Bazoum from power on July 26, West African countries imposed a slew of sanctions, closing borders and freezing financial transactions. Nigeria, which supplies most of Niger’s electricity, cut off power to Niger, crippling a landlocked nation of 25 million that relies on its coastal neighbors for imports and energy supplies.

Military action appeared a logical next step, some analysts said. Senegal, Benin and Ivory Coast said they would send troops for any military intervention against Niger’s coup leaders. But Burkina Faso and Mali, both currently suspended by the bloc of West African countries because they were taken over by military juntas, announced they would defend the mutineers.

Any military intervention would have to rely heavily Nigeria’s army, which dwarfs the others with approximately 140,000 active troops. With 220 million people, Nigeria’s population is larger than the 14 other ECOWAS countries combined.

In Niger, tens of thousands of pro-junta protesters have rallied behind the generals in the capital, Niamey, giving them a veneer of popular support. It remains unclear whether all branches of the Nigerien military back them, or whether the support extends beyond the capital.

Still, “a military intervention would be suicidal” for Nigeria and ECOWAS, said Ebezener Obadare, a senior fellow for Africa studies at the Council on Foreign Relations, worsening the country’s economy, dividing people along the border and diverting resources to warfare.

Nigeria and Niger have fought Islamist insurgents together in the Lake Chad region, home to groups affiliated with the militant group Boko Haram.

With the border between the countries closed until further notice, thousands of trucks have been stuck. Ali Saleh, a truck driver who has made cross-border trips for two decades, has been stranded for days this week and said the family-like atmosphere had vanished.

“Each side is keeping watch suspiciously,” he said.

The border regions of both countries share a similar ethnic makeup: Hausa, Fulani and other groups. “Kannywood,” the cinema industry based in the northern Nigerian city of Kano, provides households in Niger with entertainment in the Hausa language.

The ongoing stalemate has upended life near the border. Farmers who grow maize and beans in northern Nigeria can’t sell their crops in Niger; Nigerians who were studying in Niger said they don’t know when they might go back to their universities; weddings between Nigeriens and Nigerians have been postponed.

“If a fight erupts, who will be at the receiving end? Me and most of us with dual nationality,” said Ismail Yusuf, a 24-year-old textile trader in Kano, who said that his plans to marry a Nigerien woman were put on hold by both families.

In Niger, residents say food prices have soared since the military takeover, although it remains unclear whether they are primarily caused by the border closures.

Coup leaders have shunned both the threats and calls for mediation from Nigeria and ECOWAS. Twice, they refused to meet envoys from the bloc. They have also closed the country’s airspace, rerouting and delaying many flights from Europe and North African countries.

“ECOWAS has backed itself in a corner: it can’t soften its rhetoric because it would be seen as conciliatory to anti-democratic forces,” said Ikemesit Effiong, the head of research at SBM intelligence, a Nigeria-based consultancy.

The West African leaders could at least obtain safe passage for Mr. Bazoum, said Mr. Obadare, from the Council of Foreign Relations. Mr. Bazoum remains under house arrest with his wife and one of their children. Other government officials have also been arrested by the mutinous generals.

Shegun Bakari, the foreign minister of Benin, which shares borders with Niger and Nigeria, said the bloc had sent religious, military, and political mediators to Niger, with little success. But the coups cannot be allowed to continue, he said in an interview.

“We organize elections that cost a lot of money for African countries,” he said, and can’t allow “anyone who then holds a weapon to stop the democratic process.”

“It is simply not working.”

Eric Schmitt contributed reporting from Washington, and Pius Adeleye from Ilorin, Nigeria.