Number of foreign fighters in Iraq and Syria doubles

The number of foreigners flocking to the cause of militant Islam in Iraq and Syria is the largest such influx since 1945, according to a new estimate. More than 20,000 foreign fighters have joined Islamist groups in the two countries—double a previous estimate made in December 2013. About one-fifth have come from Western nations.

The influx of roughly 20,730 foreign fighters into Iraq and Syria since 2011 illustrates the ability of rebel groups like the Islamic State (ISIS) to lure supporters. ISIS grew rapidly last year and conquered large swaths of territory in a bid to establish a caliphate. Its members are known for using floggings, rape, public beheadings, and other forms of brutality to maintain control and intimidate opponents.

In its latest move, ISIS set a deadline for 9:30 a.m. EST today (sundown in Mosul, Iraq) for a prisoner swap with the government of Jordan, which is holding female terrorist Sajida al-Rishawi. If she is not released, the militants claim they will kill a Jordanian hostage, air force pilot Muath al-Kaseasbeh. On Saturday, the Islamist group claimed to have beheaded a Japanese captive, Haruna Yukawa.

ISIS has used battle videos and interviews with acolytes as part of a media propaganda scheme to summon Muslims around the world to help establish its Islamic empire. Many of those answering the call to arms have originated from Western nations, posing a challenge for law enforcement trying to detect and prevent homegrown radicalization. The emigrants have even included young women hoping to marry jihadi fighters.

So far, up to 100 foreign fighters have originated from the United States, and another 100 from Canada. Nearly 4,000 of the fighters have come from Western Europe, including hundreds from Germany and the United Kingdom, according to The International Centre for the Study of Radicalisation and Political Violence (ICSR), the London-based group that tracks foreign fighters and compiled the new estimates.

But most of the foreign fighters have come from Middle Eastern and North African nations, such as Tunisia, Saudi Arabia, Lebanon, and Morocco. Up to 1,500 have come from Russia. The Washington Post put the figures on a map of the globe, vividly illustrating the surge in foreign Muslims answering the call to jihad.

The total, notes ICSR, represents the “largest mobilization of foreigner fighters in Muslim majority countries since 1945.”

The number of foreign fighters currently on the ground in Iraq and Syria is less than 20,000, according to ICSR. Its figures are cumulative, and include fighters that have since been killed or returned home—probably 15 to 40 percent of the total.

Previous estimates from ICSR showed militant Islamic groups around the world killed 5,042 people in November 2014 alone—168 per day. The deadliest group was ISIS, responsible for nearly half the killings.