About 8,000 people have streamed into the Spanish city of Ceuta from Morocco in the past two days in an unprecedented influx, most of them swimming around breakwaters and across the border to reach the Spanish enclave in North Africa.

The surge has strained relations between Morocco and Spain, with Spanish Prime Minister Pedro Sánchez canceling a trip to Paris to make an unscheduled visit to Ceuta, where Spain has deployed military reinforcements and police along the border. Here's a look at what's going on:

WHERE IS CEUTA?

Ceuta is a coastal city in North Africa that has belonged to Spain since the 16th century. Like Melilla, another Spanish possession on the Moroccan coast, Ceuta in recent decades has become a flashpoint for migrants from Morocco and sub-Saharan Africa seeking to enter Europe.

Last year about 2,200 people crossed into Ceuta and Melilla by scaling border fences or swimming from the Moroccan side. Ceuta has a population of 85,000 and is connected to mainland Spain by ferry services across the narrow Strait of Gibraltar.

WHAT IS HAPPENING IN CEUTA?

Migrants regularly make it across the border in small numbers, but the scale of the crossings this week is exceptional. Thousands of people were able to reach the border area without being stopped by Moroccan authorities.

About 8,000, including 2,000 believed to be minors, reached Ceuta in the past two days by swimming or paddling in small boats around breakwaters separating the two countries. Most were Moroccans, though there were also migrants from sub-Saharan Africa.

Spain deployed troops and armored vehicles to the border on Tuesday, rounding up migrants on a beach and sending many of them back to Morocco through a gate in the border fence. The Red Cross says one young man died and dozens were treated for hypothermia.

WHAT IS BEHIND THE SURGE?

Morocco has said little about why it relaxed the border controls, though it was widely seen as retaliation against Spain for having allowed the leader of a militant group, Brahim Ghali, to receive medical treatment in a Spanish hospital.

Two Moroccan officials made that link in public comments on Wednesday. Ghali heads the Polisario Front, which is fighting for an independent Western Sahara, a former Spanish colony that Morocco annexed in the 1970s. He was hospitalized in the Spanish city of Logrono last month in a move that angered Morocco’s government, which warned there would be “consequences.”

Some experts say the issue goes beyond Ghali and that Morocco wants Spain to support Morocco's sovereignty over Western Sahara, like the U.S. did under the Trump administration last year.

WHAT HAPPENS TO THE MIGRANTS NOW?

Spain's Interior Ministry said about half of those who made it across have already been sent back to Morocco.

Under a three-decade-old agreement between the two countries, Spanish authorities can return adults who cross the border irregularly. On Tuesday, Spanish soldiers could be seen directing migrants toward a border gate, in some cases hitting them with batons to make them hurry up.

An AP reporter saw several children among those being pushed back, even though the Spanish government claimed that no unaccompanied minors were being returned. Many of the unaccompanied minors were being held in quarantine in warehouse shelters run by the Red Cross.

WHAT ARE THE WIDER IMPLICATIONS FOR SPAIN?

The developments in Ceuta have become one of the biggest crisis in relations between Spain and Morocco since 2002, when a territorial dispute erupted over an uninhabited island off the Moroccan coast. It represents a humanitarian, diplomatic and political challenge for Sánchez's government.

In recent years Spain has seen spikes in migrant arrivals on its southern coast as well as in the Canary Islands, sparking concerns over migration that have helped fuel the rise of Vox, a far-right party that entered Parliament in 2019.

Vox was quick to blame the situation in Ceuta on the government’s “inaction" and its leader visited the city on Tuesday.

HOW DOES THIS AFFECT MIGRATION ACROSS EUROPE?

Other European Union nations are watching the developments in Ceuta carefully. Since Europe's migrant crisis in 2015, the bloc has tried to reduce the flow of irregular migrants to Europe in part by seeking agreements with transit countries — including Morocco, Turkey and Libya — to hold back migrants.

The situation in Ceuta and a similar crisis on Turkey's land border with Greece last year show how such deals can give transit countries plenty of leverage over the 27-nation EU.

The bloc's Home Affairs Commissioner Ylva Johansson called the Ceuta influx “worrying" and noted that Spain's border with Morocco is also the EU's external border. She urged Morocco to prevent more people from crossing it irregularly.

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Associated Press writer Aritz Parra contributed to this report.

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Follow AP’s global migration coverage at https://apnews.com/hub/migration