JOHANNESBURG — African borders are often a tangle of razor wire, with soldiers at crossing points checking papers, waving on those with valid IDs and turning back the rest.

Even so, an estimated 1,200 undocumented migrants every night cross the Limpopo River, South Africa’s equivalent of the Rio Grande separating the U.S. and Mexico. Most are from Zimbabwe, where shortages of food and fuel and an unemployment rate of more than 80% have sparked a steady exodus to the more stable and more prosperous South Africa.

A milewide “dead zone” fenced on both sides stretches east and west from Beit Bridge, the sole legal crossing between the two countries. Police and military are on patrol day and night.

Some of those caught say they are beaten and sent back. Others pay bribes and go on to Johannesburg, the financial hub of South Africa and a city whose economy is larger than the whole of Zimbabwe‘s.

The beefed-up border is a startling sight for a continent where such barriers not long ago were the rare exception rather than the norm.

Human rights groups say borders across the continent are becoming harder to cross, with guns, dogs and government policies determined not just to keep out illegal crossers, but also to return those who still have no status after entering up to a decade ago.

This year, Angola has deported an estimated 330,000 “irregular migrants” who have fled ethnic violence in neighboring Congo. The United Nations warned of a “humanitarian catastrophe,” and the Norwegian Refugee Council office in Congo termed it “a full-blown emergency.”

But the Angolan government trucked or marched refugees to the border, where they were pushed across.

Equatorial Guinea sparked a diplomatic crisis this month with its neighbor Cameroon after announcing plans to build a wall between the two countries. The former Spanish colony — Africa’s fourth-largest oil producer — says Cameroon allows Nigerians and others to transit its territory and cross the border in search of work.

Freedom House ranks both countries among the world’s least democratic nations. Paul Biya of Cameroon is the world’s longest-serving head of state. When he was first offered the post of prime minister in 1975, Gerald Ford was in the White House and Elvis Presley was on stage in Las Vegas.

In Malabo, the capital of Equatorial Guinea, President Teodoro Obiang Nguema Mbasogo took power in 1979 when he mounted a coup overthrowing his uncle, who was executed by firing squad. The press is under state control, and a French court found Mr. Nguema’s son guilty in 2017 of embezzling $175 million and using the money to buy villas, cars and luxury goods. He was given a suspended sentence.

Politics and immigration

In this case, a separatist movement has made the drift of migrants worse. In 1961, two colonies, one British and the other French, were joined to create what is now the Republic of Cameroon, but the English-speaking minority claims it is the target of discrimination and abuse and has long sought to break away as the “Republic of Ambazonia.” The people say they are ethnically closer to the tribes of eastern Nigeria.

Secessionist rallies in recent months led to a crackdown, with troops burning homes and firing on civilians, many of whom fled to neighboring states.

But Gen. Rene Claude Meka, Cameroon’s army chief of staff, called plans for a border wall an act of aggression. In July, he toured the 110-mile border and insisted the project would “not be tolerated.”

When Equatorial Guinea’s ambassador was summoned to explain what Mr. Nguema had in mind, the envoy insisted the wall project would proceed but denied rumors that a team had already used beacons to mark a line where bricks would be laid.

Analysts say the wall is more a sign of Mr. Nguema’s insecurity and that, over the years, attempted coups have fostered a mood of paranoia. In December 2017, dozens of “mercenaries” from other African states were arrested near an entry point with Cameroon, allegedly in possession of rifles, ammunition and rocket launchers. Mr. Nguema closed the border for six months.

Human Rights Watch and Amnesty International have described the widespread use of torture in Equatorial Guinea, and critics say the 2017 arrests were a sham used to harass opponents of the regime. Across the country, hundreds were taken into custody.

But whether crossing into Cameroon, Zambia, Kenya or Mozambique, there are signs of a new order: more troops, more border checkpoints, more people told they can no longer pass lines drawn in colonial times that divide tribes, ethnic groups and traditional communities.

Police in Johannesburg in the past month have taken to sealing off streets in poor areas, typically at rush hour, blocking doorways and then searching cars and frisking pedestrians. Anyone without a South African identity book or a refugee permit is detained.

The government has said the crackdown is focused on counterfeit goods and unlicensed weapons, but migrants from Congo, Nigeria, Zimbabwe and more distant places such as Syria and Bangladesh say they are the real targets.

Zimbabweans face a number of physical barriers escaping their country’s woes. Botswana has erected a 310-mile-long, 6-foot-high electrical fence, purportedly to keep out smuggled Zimbabwean cattle that could spread foot-and-mouth disease. South Africa maintains an electrified fence along parts of its border with Zimbabwe and Mozambique.

There is no talk of a wall along the Limpopo, but if Equatorial Guinea gets its way, other leaders may come under pressure from voters who say foreigners are taking jobs and straining hospitals, classrooms and social services.

In Johannesburg, Enoch Gumede, a Zimbabwean math teacher caught in a roadblock and detained for five hours even after he produced a valid work permit, said he has advised students and their parents at the community school where he works to arrive early.

“A lot of our learners are not born here,” he said. “The police act between 8 a.m. and midmorning, so we now open the school early and tell those who may be vulnerable to get in by sunrise.”

He said families were also less inclined to go home during holidays.

“Over Christmas, they would visit relatives in Malawi, Mozambique, Zimbabwe, but it has become difficult,” Mr. Gumede said.

“It’s not that you can’t cross without papers, but when controls are tightened, the price of a bribe goes up. Many can no longer afford it.”

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