Thousands of migrants in recent days have crossed onto a tiny peninsula on Morocco’s coast that Spain has governed for centuries, aggravating tensions over a border that has long troubled the relationship between the governments of Madrid and Rabat.

At least 18 people have died so far in their attempt to reach Spanish soil, a government spokesman for Spain said on Thursday.

The peninsula, Ceuta, is one of only two cities on the African mainland that are officially part of Europe. In search of better economic prospects, thousands of migrants have scaled a 20-foot-high border fence into Ceuta or entered the territory by sea in mass crossing episodes over the years.

But so many migrants made the passage on Thursday that it prompted Spain to mobilize its military and ignited backlash from the country’s right-wing opposition, which accuses the government of Prime Minister Pedro Sánchez of being lax on border control.

We covered the news here. Here’s a guide to why Ceuta matters.

Why is Ceuta a crossing hot spot?

A six-mile-long peninsula on Morocco’s northern coast, Ceuta is a Spanish enclave fortified by a double fence and a Spanish police and paramilitary presence.

The city, along with Melilla, another autonomous Spanish city further east along the same coast, have the European Union’s only land borders with Africa.

That means both cities are places where migrants seeking better livelihoods, many of them from Morocco, frequently attempt to cross into Europe.

More than 10,000 people came over the border into Ceuta over two days in 2021, for example. In 2022, at least 23 migrants died in a stampede during an attempted mass crossing in Melilla. Many others have died at sea.

To get in, some migrants swim around the fences. Others take short, illicit boat trips to Ceuta from Morocco. But mostly, they run and climb the fence, or use bolt-cutters to cut through it, and are often spotted by motion detectors and guards in observation towers in the process.

(Here’s a fascinating story about Ceuta’s soccer team, which unpacks the territory’s relationship with Spain.)

What happened there this week?

Some 1,500 migrants had already entered Ceuta in recent days. But on Thursday, Fernando Rodríguez Contreras, a Spanish government official, said that what may have been tens of thousands of people crossed over, a working estimate that could not be confirmed. At least 18 of them died in the attempt, a Spanish government spokesman said.

Spain’s government said in a statement that it was dispatching military platoons to enforce border security, increasing air and naval support and sending diving teams to patrol the waters. The European Union offered to send Spain support from Frontex, its border and Coast Guard agency, to reestablish order.

Why does this mass crossing matter?

The African mainland border has been a major source of domestic political tension for the left-wing government of Mr. Sánchez and a diplomatic pain point between Morocco and Spain.

Domestically, Mr. Sánchez has sought to project an image of Spain as the West’s new melting pot. He has embraced immigrants, especially Latin Americans, who speak Spanish.

But on Ceuta, Mr. Sánchez has drawn criticism from anti-immigrant and conservative political opponents, who accuse him of being lax on border control — as well as pro-immigrant advocates who criticize the government for harsh immigration enforcement tactics.

Right-wing critics seized on the moment Thursday, blaming Spain’s new plan to give undocumented migrants already in the country a path to legal status for attracting illegal arrivals.

“This is an invasion,” said Santiago Abascal, the leader of the far-right party Vox, who accused Mr. Sánchez’s government of purposefully inviting in the migrants, who he suggested, without evidence, were criminals, rapists and potential left-wing voters.

The government, however, made clear for its part that the high volume of illegal crossings was not part of its vision.

On the international front, Spain is eager to keep up good relations with Morocco, in part to maintain its cooperation in deterring migrant arrivals. After the major crossing episode in 2021, Spanish government officials said that Morocco saw the migrants as a kind of currency, leveraging control over their movement across the border to extract financial and political prizes from Spain.

Hours after the migrants began pouring into Ceuta that year, Spain approved 30 million euros, about $37 million, in aid to Morocco for border policing.

It was unclear on Thursday evening what had prompted the latest amassing of people in Ceuta. Analysts said it was too soon to draw conclusions but that Morocco may have been upset about Spain’s recent effort to deepen ties with Algeria.