One of the policy concerns about violent conflicts in Afghanistan, Iraq, Libya, Mali, Pakistan, Russia’s North Caucasus region, Somalia, and the ongoing war in Syria has been that these struggles could attract and breed foreign fighters. Yet  we know relatively little about the ways in which foreign fighters, once they arrive, influence the struggles they join. I argue, in my article in the spring 2014 issue of International Security, that foreign fighters may actually weaken the rebel movements they come to support.

Foreign fighters, or transnational insurgents, are fighters without affiliation to a formal army who voluntarily join the rebels in a civil war outside their own home country. Contemporary policy debates focus on Islamist insurgents, but research by David Malet shows that foreign fighters can have other ideological or ethnic attachments to the struggles they join (think of the International Brigades in the Spanish Civil War).

Western governments are concerned about foreign fighters for a number of reasons. For starters, they worry that by participating in wars abroad, foreign fighters will radicalize and gain skills they can put to use back home. However, research by Thomas Hegghammer, a leading specialist on violent Islamism, questions this concern, and others suggest it might even lead to misplaced policies.

Another worry, shared with governments of states fighting insurgencies within their borders, is that foreign fighters bring along manpower, weapons, know-how and access to financial resources, all of which may strengthen the domestic rebels and potentially prolong and spread the war. As noted by Peter Bergen and Alec Reynolds about Iraq in the early 2000s, “The Jordanian Anu Musab al-Zarqawi, another alumnus of the Afghan war, is perhaps the most effective insurgent commander in the field.” For domestic rebels, the added expertise is precisely why they might welcome the outsiders in the first place.

Yet, as we have come to see in Syria, domestic rebels and the population may grow skeptical of their foreign helpers. Indeed, it is not a given that foreign fighters strengthen the domestic rebel movements they join. While foreign fighters may boost a domestic rebel movement’s coercive force through added resources and expertise, they can also weaken the movement’s organizational cohesion and ability to mobilize supporters by introducing new ideas about goals and tactics.

As I show in an in-depth study of the influence of foreign fighters on the Chechen separatist movement, the entry of new goals and tactics can cause divisions within the movement – and even outright defection – if local resistance leaders are not on board. The introduction of new goals and tactics can also make it more difficult for the movement leaders to garner public support.

Building on insights from research on peaceful transnational relations of how ideas spread, I suggest that central to whether and how foreign fighters strengthen or, conversely, weaken the rebel movement they join is the ability of domestic resistance leaders to foster local acceptance of the ideas brought along by the outsiders.

In Chechnya, as Cerwyn Moore and Paul Tumelty have shown, the first foreign fighters, mainly from the Middle East, entered in January 1995, a couple of months into the 1994 to 1996 war against the federal government in Moscow. By 1996, foreign fighters had set up training camps in the republic’s mountainous eastern part. They also came with new ideas of what the struggle was about and how it should be fought. Although the change cannot be attributed exclusively to the foreign fighters, they certainly contributed to transforming a nationalist struggle for Chechen independence into one dominated by the goal of establishing an Islamic emirate in the Caucasus. The same goes for tactics. The responsibility for the turn to radical tactics involving civilian targets, such as suicide terrorism, lies not only with the foreign fighters, but the timing suggests the foreign fighters played a central role.

However, these new ideas about goals and tactics got only a lukewarm reception from the local population. Some locals even claimed that the views imposed on them by the outsiders contradicted their local traditions and values just as much as the views imposed on them by the Russians. Some local resistance leaders tried to make the case that these ideas were not wholly in contradiction with local ideas about what the struggle was about and how it should be fought, but other resistance leaders outright rejected them. That is, even though the foreign fighters who arrived in Chechnya may have strengthened the local resistance movement’s coercive force in their fight against Moscow, they ultimately helped weaken the movement. The foreign fighters contributed to a formal split of the original movement into an Islamist and a nationalist branch, as well as outright defection to the Russian side under Akhmad Kadyrov, the father of the republic’s present pro-Russian president, Ramzan Kadyrov.

Similar dynamics seem to be at work in Syria. News reports suggest that not only has the influx of jihadists led to outright clashes between foreign and local groups, but the influence of the jihadists, some of whom have tried to impose strict Islamic codes, have alienated the local residents. A fuller understanding of how the foreign fighters have shaped the Syrian struggle requires detailed analysis of how local resistance leaders have (or have not) tried to “localize” the outsiders’ ideas.

The point is, it might well be that foreign fighters prolong civil wars and contribute to their spread across borders. But not necessarily because they strengthen the domestic rebel movement, rather because they weaken it. Indeed, given the growing body of work pointing to how weak and fragmented rebel and opposition movements are likely to foster violence, lengthen conflict duration and complicate mediation and negotiation efforts, the subversive effect that foreign fighters may have on a domestic rebel movement’s strength is potentially bad news all around. It is bad news not just for the domestic rebels, but also for states fighting insurgencies within their borders and for (international) actors engaged in conflict resolution.

To the degree that it is in the power of local resistance leaders to “sell” goals and tactics brought along by outsiders to the local population – and local support matters for the domestic movement’s strength – policymakers concerned about foreign fighters should focus their efforts on alleviating both the wish and need for such outside help among local resistance leaders.

Kristin M. Bakke is a senior lecturer in the department of political science at University College London.