On May 17, more than 6,000 people swam, floated or scaled a pair of 32-foot-high border fences to cross from Morocco into neighboring Ceuta, an eight-square-mile Spanish-owned city on Africa’s northern coast. Critically for those seeking to enter, Ceuta is inside European Union territory. According to Spanish authorities, it was the largest single-day influx of unregulated migrants in the country’s history. At least 2,000 more followed the next day.

This wasn’t an accident. Apparently, Morocco engineered this mass cross-border movement to punish and coerce Spain. Video footage appeared to show Moroccan border guards opening fences to the Spanish enclave and allowing people through.

Nor was this unusual. Strategically engineered migration is far more common than most people realize. At any given time, somewhere in the world, leaders inside or outside governments are likely manipulating migrants and/or refugees to pursue political, military or economic objectives. Here’s what we know.

Weapons of mass migration

When governments or others “weaponize migration,” they manipulate or exploit the movement of people — or threaten to do so — to achieve a strategic objective. Morocco unleashed a migrant surge on Spain in retaliation for Spain’s decision to admit Brahim Ghali, leader of the Polisario Front, for medical treatment. The Polisario Front is a separatist movement battling Morocco for Western Saharan independence.

In the days before opening the border, Morocco indicated that it was planning an “appropriate response” to Spain’s decision to admit Ghali. After the border was again sealed, Morocco’s ambassador to Spain warned that the crisis could worsen, depending on how Spain deals with Ghali going forward. With this, Morocco signaled it wants not just to punish Spain’s decision to admit the rebel leader but also to influence Spain’s future actions.

As my research has detailed, governments and other non-state actors have long weaponized cross-border population movements for their own purposes. This most often entails threatening either to overwhelm a target’s physical capacity to accommodate a mass migration or its society’s political willingness to do so. Such threats can be surprisingly potent.

For instance, former Cuban president Fidel Castro successfully wielded this tool against the United States at least three times to extract political concessions. This happened most famously during the 1980 Mariel boatlift, when 125,000 Cubans, including some who were criminals or mentally ill, were dispatched to Florida from the island nation; and arguably still more successfully during the 1994-1995 balseros crisis, when Castro again opened Cuba’s borders after the United States failed to meet his demands. Similarly, in 1994, exiled Haitian President Jean-Bertrand Aristide refused a U.S. request that he dissuade Haitians from heading to the States, hoping that the United States would unseat Haiti’s reigning military junta and facilitate his return to power.

Why weaponize migration?

My research identifies four types of strategic engineered migration. These aren’t mutually exclusive; several goals can coexist.

  • In dispossessive engineered migrations, a government or non-state actor displaces a group or groups to acquire the territory or property of those displaced or eliminate them as a threat to its own ethnopolitical or economic dominance.
  • In exportive engineered migrations, a government aims either to shore up its political position by expelling political adversaries or to discomfit or destabilize one or more foreign governments with the sometimes quite costly consequences of mass movements.
  • In militarized engineered migrations, people are displaced — usually during armed conflict — so that one side can gain military advantage. Either governments or rebel groups may be trying to disrupt or destroy their opponents’ command and control, logistics, or movement capabilities by undermining their civilian support structures, creating logjams and causing humanitarian crises. Or they may do so to seize the displaced people’s assets or forcibly conscript them.
  • In coercive engineered migrations, a government or other actor deliberately creates or manipulates a mass migration to win concessions from a target state or states.

Morocco’s actions last week involved “coercive engineered migration,” a relatively common strategy. Since 1951 alone, coercive engineered migrations have been attempted at least once a year, on average. Of the cases my research has uncovered, more than 60 percent were launched for political reasons, as in Ceuta. Approximately 30 percent were launched to achieve military objectives. And roughly 50 percent were aimed at achieving an economic goal like foreign aid. This totals more than 100 percent, because often those who use this tool are pursuing more than one objective simultaneously.

The methods of weaponized migration

Leaders can actively engineer migration, using the military to generate mass movements. For instance, in a failed attempt to coerce NATO to halt its 1999 bombing campaign, Yugoslav President Slobodan Milosevic used the army, special police units and armed Serb civilians to expel Kosovar Albanians, particularly in areas with pronounced rebel Kosovo Liberation Army presence. Or they can passively engineer mass movements, leaving normally sealed borders unguarded — as Morocco did with Ceuta.

Sometimes threats alone are enough. Former Libyan leader Moammar Gaddafi repeatedly made dramatic promises to “turn Europe black” if the E.U. failed to meet such demands as lifting arms sanctions, providing military and economic aid, or ending support for domestic opposition groups. Gaddafi succeeded, at least partly, in 2004, 2006, 2008 and 2010 — but fatally overplayed his hand in 2011, when he was deposed and killed in a NATO-led military intervention.

Other times, leaders exploit and manipulate displaced people who are already on the move. For instance, as refugees from the Syrian civil war and other migrants have fled north over recent years, Turkey’s president, Recep Tayyip Erdogan, has threatened to flood the E.U. with displaced people to extract aid, promises to restart talks on E.U. accession, and more, with varying levels of success.

Coercive engineered migration can be strikingly effective

In about three-quarters of the cases I identified, coercers achieved at least some of their objectives. In more than half the cases, coercers obtained all or most of what they sought. Weaponized migration appears to be more effective than either economic sanctions or coercive diplomacy. It’s also significantly harder to manage and control, however, and therefore, rarely a strategy of first resort.

Whether Morocco gets what it wants, its true victims are the migrants and refugees themselves. Unfortunately for these victims, we can expect more weaponized migration in the future.

Kelly M. Greenhill is 2020-21 Leverhulme Trust Visiting Professor at SOAS University of London, currently on research leave from Tufts University and Harvard Kennedy School of Government’s Belfer Center.