An armed man, believed to be an Islamic state militant, pictured near the northern Syrian town of Tal Abyad as seen from the Turkish border town of Akcakale, southeastern Sanliurfa province, January 29, 2015. REUTERS/Osman Orsal The Islamic State lost a crucial "back door" into Turkey earlier this week when Kurdish forces ran the militants out of Tal Abyad, cutting off the terror army's supply route for weapons, cash, and foreign fighters into Syria.

"This is a devastating blow to ISIS' operations," Jonathan Schanzer, vice president for research at the Foundation for Defense of Democracies, told Business Insider. "ISIS lost its most important back door to Turkey."

An unnamed US official told The Wall Street Journal something similar.

“[ISIS] was focusing so much on Ramadi and Beiji [in Iraq] they lost sight of the back door,” the official said. “It is a pretty significant victory.”

The relaxed border policies Turkey adopted between 2011 and 2014 enabled extremists who wished to travel to Syria and join the rebels in their fight against the regime of Syrian president Bashar Assad.

Turkey officially ended its open-border policy last year, but not before its southern frontier became a transit point for cheap oil, weapons, foreign fighters, and pillaged antiquities. Smuggling networks along the nation's 565-mile border with Syria managed to emerge and flourish while the policy was in place, with the Islamic State (aka ISIS, ISIL, or Daesh) being the main beneficiary recently.

However, "Syrian Kurds now control a contiguous 245-mile stretch of territory from Kobani east on the Syrian-Turkish border," The Journal reports.

Pipes, ammonium nitrate, and other bomb-making materials were being transported across Turkey's border into Tal Abyad by agents of ISIS while Turkish border guards looked the other way, Jamie Dettmer of The Daily Beast reported.

Schanzer noted that the fall of Tal Abyad was a sharp blow to ISIS operations, but it was by no means fatal, as ISIS has multiple "back doors" to Turkey and will likely try to open new supply lines in the coming weeks and months.

The US has tried to cut off ISIS' main sources of revenue with little success in the past: ISIS is one of the most well-funded terrorist organizations in history thanks to the tax base it has managed to establish in its vast swaths of conquered territory in Iraq and Syria — territory captured, in part, using the weapons and fighters flowing across the Turkish border.



ISIS control and attack zones as of June 1. REUTERS Successfully cutting off these supply lines could be the beginning of a strategy to cripple ISIS, but it would require action on both sides of the border.

"The Turks have been less than cooperative," Schanzer noted. "The border between Turkey and Syria has remained incredibly loose."

Still, "should anti-ISIL forces continue to hold the city, there is the potential for a significant disruption of ISIL’s flow of foreign fighters, illicit goods, and other illegal activity from Turkey into northern Syria and Iraq," Edgar Vasquez, a spokesperson for the US State Department’s Near Eastern Affairs Bureau, told The Journal.

This is especially true given ISIS' financial dependence on the resources it steals from conquered territories. Without fighters and weapons, ISIS cannot expand, and without expanding, it cannot make money.

People wait at the Akcakale border gate in Sanliurfa province, southeastern Turkey, where Islamic State militants have been in controlled the Syrian side of the gate before this week. REUTERS/Osman Orsal

In this way, one of ISIS' greatest assets — its ability to plunder resources — might also be one of its weaknesses.

"The weakness of ISIS' strategy is that in order for it to continue to bring in additional resources, it needs to continue to expand," Schanzer added. "But the further they stray from their core logistical bases, the less able they will be to defend them."

Michael B. Kelley contributed to this report.

