The Mosul crisis highlights how ISIS has established a potent cadre of foreign jihadists who freely operate across the rapidly disappearing Iraq-Syria border.

Much attention has rightfully been given to the massive wave of Sunni foreign fighters entering Syria to fight the Assad regime in the past two-and-a-half years, but an overlooked aspect of the war's spillover is the effect on neighboring Iraq. In particular, a growing number of foreign fighter contingents have returned there under the banner of the Islamic State of Iraq and al-Sham (ISIS), determined to fight the Maliki government and the Sunni sahwa (awakening) movement that joined the United States last decade in standing up to "al-Qaeda in the Land of Two Rivers" (popularly known as AQI, one of ISIS's prior names). This trend -- in conjunction with Iraqis fighting alongside ISIS in Syria and the group steadily seizing territory in western Iraq and eastern Syria -- has effectively melted away the border. ISIS operatives now operate in both countries and view crossing the border as nothing more than going from one province in their "Islamic State" to another.

This week's ISIS takeover of Mosul can be viewed as the crest of a second wave of foreign fighters entering Iraq. The first such jihadist wave occurred during the height of the Iraq war last decade, when an estimated 4,000-5,000 foreign fighters arrived over a five-year period (mostly Saudis, Libyans, and Syrians) and were largely quelled by the local sahwa movement and the U.S. "surge."

After the backlash against AQI several years ago, the organization decided to "Iraqify" itself, believing that locals viewed it as a foreign entity since many of its leaders and members were from other countries. As part of this decision, the group rebranded itself as the Mujahedin Shura Council and later the Islamic State of Iraq, which in turn was changed to ISIS in April 2013 to incorporate its new area of operations ("al-Sham" includes Syria).

The creation of ISIS and its official entrance into the Syrian arena had major reverberations back in Iraq, including an uptick in the death toll from ongoing violence. According to Iraq Body Count, a website that has collated civilian deaths since the start of the 2003 war, approximately 389 deaths occurred per month during the post-sahwa/surge period (October 2008-April 2013). Yet from May 2013 to May 2014, the rate rocketed to 1,029 deaths per month, and June is projected to outpace that average. This upward trend is in line with datasets released by the UN and Agence France Presse, albeit with differing numbers due to their distinct methodologies.

Indeed, the move to Syria not only provided ISIS with a safe haven, but also allowed it to more easily recruit foreigners to its cause and open new lines of communication for transferring money and weapons between the Syrian and Iraqi battlefields. Despite the group's past rebranding and "Iraqification" efforts, the fact remains that foreign fighters tend to have superior fighting skills and are therefore force-multipliers on the battlefield and in training new recruits. Tapping these foreign sources of power, financing, and weapons helped ISIS reignite its insurgency in Iraq at a much higher level. Moreover, when ISIS split from al-Qaeda's official Syria branch, Jabhat al-Nusra, in April 2013, many of the latter group's foreign fighters joined ISIS, and the networks that had funneled jihadists to JN began to send them to ISIS instead.

Beyond gaining new recruits and resources in Syria, ISIS also carried out audacious jailbreaks at Iraq's Abu Ghraib prison and other facilities, freeing numerous fighters who had been arrested during the sahwa/surge, many of whom were foreigners. Although the group's claims about the number of freed prisoners are exaggerated, credible estimates suggest that more than 500 fighters were able to return to the battlefield and provide their expertise to ISIS. This reemergence of hardened fighters who had been involved with the organization at the height of the Iraq jihad last decade helped push ISIS to another level militarily. Taken together, the various trends that have strengthened the organization over the past year also explain the reemergence of foreign fighters in Iraq.

THE NEW CONTINGENT

Similar to AQI's original practice of posting official martyrdom notices, ISIS began doing so itself earlier this year, highlighting its comfort in sharing such information. Since early March, the group has released 201 martyrdom notices on its official province-level Twitter accounts for foreign fighters killed in Iraq (most of the notices were also posted on online jihadist forums). Although some of these notices were for individuals who died as far back as September 2012, the vast majority were for deaths that occurred after April 2013.

Since this information is self-reported by ISIS, and because the group continues to release older notices, the actual number of foreigners who have died in Iraq is likely higher. Further, some notices do not name a specific country of origin, instead using phrases such as "al-Shami" (which could denote anyone in the Levant) and "al-Muhajir" (meaning simply "emigrant"). In any case, this year's jihadist death toll is set to exceed last year's -- if the current pace continues, some 233 foreign fighters will have been killed in Iraq by the end of 2014, or two-and-a-half times more than 2013. And the pace will likely accelerate given the increased fighting.

Similar to last decade, Saudis are well represented in ISIS martyrdom notices (with 38 dead), as are Libyans (10 dead). And many of the 18 "unknown" cases are likely Syrians. The biggest change is the enormous growth in the Tunisian (57 dead) and Moroccan (27) contingents. And while the majority of fighters hail from the Arab world, the notices also name individuals from the Balkans, the Caucasus, Central Asia, the Horn of Africa, North America, and Western Europe. Among the eight Westerners represented, three were from Denmark, three from France, and one each from Canada and Norway.

ISIS classifies its "martyrs" by wilayah (provinces), largely using the same names as the Iraqi government's muhafaza (governorates), though it calls Babil governorate "Wilayat al-Janub" (the south province). Based on these notices, the majority of foreign fighters killed in Iraq have fallen in the ISIS stronghold of Anbar (56 deaths), in parts of northern Baghdad province (51), and in Diyala province (41). The recent takeover of Mosul could increase the death toll in Ninawa province, where 29 fighters have already been killed.

RAMIFICATIONS

The return of foreign fighters to Iraq and the fact that ISIS now controls more or less contiguous territory has allowed it to deploy fighters in different districts of its "Islamic State" irrespective of the Iraq-Syria border. Many fighters have fought in Syria and died in Iraq, and vice versa. While observers tend to focus on the "Syria foreign fighter issue," the problem actually spans two countries -- Iraq is now a key part of the same quandary that Western leaders have been attempting to figure out in Syria for some time. For budding jihadists worldwide, the continued battlefield victories registered by ISIS and the perception that the "will of God" is on its side against numerically superior enemies will only enhance the prestige of joining the group and furthering its goals.

Aaron Y. Zelin is the Richard Borow Fellow at The Washington Institute.