Sunni Muslims particularly in the Arab world.

The numbers are still not minute available, but the head of the National Counterterrorism Center testified in February 2015 that more than 20,000 foreign fighters from 90 countries, at least went to fight in Iraq and Syria.

Only 3,400 of those are from the United States and Western Europe, while the rest came from Muslim countries, especially from the Arab world.

There are a few number of the non-participating countries, where jihad hotbed is there for a long time, Saudi Arabia again is a trusted supplier of jihadist fighters, so does far countries from Syria and Iraq, such as Tunisia, Libya and Morocco.

But, what happens when these fighters return home?

A research report for the Brookings Institution wrote by Daniel Byman said that foreign fighters who acquire combat experience in Iraq and Syria constitute double jeopardy. Many of those who go to war as veteran combatants will return to fight back and steadfast in facing danger, and skilled in the use of weapons and explosives -an ideal martial to recruit terrorists.

More important, their worldview may change. While in the conflict zone, they will form networks with other radicals, embrace techniques like suicide bombings and beheadings, and establish ties to jihadists around the world, making them prone to further radicalization and giving them access to training and weapons they might otherwise lack.

Several countries in the Arab world, notably Libya and Lebanon, face considerable risk of bleedout from returning fighters and several more face more modest dangers, particularly Algeria, Egypt, Tunisia, and Yemen. However, there is no simple model of bleedout, in part because the groups in Syria and Iraq, and the global jihadist movement in general, are divided as to focus and strategy.

In addition, different countries have different mitigating factors. In particular, the presence or absence of strong and focused security services will have a profound impact on the risk of bleedout. Although many countries are at risk of violence, the strategic impact of returning fighters is likely to be more limited. Militarily and tactically they can create new groups or strengthen existing ones; however, their ambition, regional focus, lack of discipline, and brutality often mean they create more enemies than they vanquish and anger local populations, strengthening the government’s hand.

While it is likely that they will use terrorism, it will primarily be locally and regionally focused, with international terrorism probably less of a priority. Terrorism against Western targets in the region is also likely to grow. Those who fight with the Islamic State imbibe its hostility to the West, both as a military enemy but also as a presence that ideologically is opposed to “true” Islam. Kidnapping of Westerners for ransom is also likely, largely for financial reasons but also because of the publicity such actions bring.

Arab states can reduce the risk of bleedout by hindering the travel of volunteers and constraining their ability to organize, countering the narrative more effectively by stressing the internecine nature of the violence in the Sunni Muslim community, and developing effective deradicalization programs. Regional and international cooperation to monitor and disrupt travel is also valuable. On the other hand, these regimes are likely to take advantage of the jihadists’ presence to gain more support from the United States, delay democratic reforms, and crackdown on non-jihadist opposition.

Returning foreign fighters offer new opportunities to gather intelligence that regional services may be slow to collect and process. In particular, the heavy use of social media like Facebook and Twitter by today's foreign fighters is a source of vulnerability , allowing for easy collection and knowledge of broader networks as well as real-time information about the movements and activities of fighters. Improving intelligence sharing and offensive counterintelligence is therefore critical to mitigating the terrorism threat. This is where the United States can play an important facilitating role by bringing the different intelligence services together and facilitating the flow of information, particularly in cases where suspicions (or just politics) limit cooperation.