In October 2007, U.S. coalition forces conducted a raid near Sinjar, along Iraq's Syrian border. They found one of the most important caches of the war: papers.

The raiders uncovered dossiers — personal records and questionnaires — on foreign fighters who had traveled to Syria and then crossed the border into Iraq to fight for al Qaeda. The dossiers had been compiled by an al Qaeda Iraqi affiliate originally known as the Mujahidin Shura Council, which later morphed into the self-proclaimed Dawlat al Iraq al Islamiyya (the Islamic State of Iraq).

The document cache gave U.S. intelligence officers keen insight into the workings of one of al Qaeda's most important tools: the pipeline.

The pipeline carried foreign fighters from many nations, though most came from Saudi Arabia, Libya, Syria, Tunisia, Algeria, Morocco, Jordan and Yemen. Their ranks included students, teachers, businessmen, lawyers, engineers, ex-military, artists, fitness trainers, massage therapists, bomb-makers and farmers. Some signed up to fight in the fields. Others were designated for "martyrdom" as suicide bombers.

Foreign fighters were a small but critical part of the insurgency. The pipeline provided needed, dependable reinforcements, a handy supply of martyrs, and a powerful propaganda tool, as the foreigners' presence linked the struggle in Iraq to the broader global Islamist insurgency. Moreover, those who survived could move on to other conflicts as trained, credentialed, proven leaders.

The Iraq pipeline was not the first. When the Russians invaded Afghanistan, many young men from the Middle East made their way into Pakistan and, from there, joined the ranks of the Afghanistan Mujahideen. Among them was a recent college graduate who had worked in his family's construction business, Osama bin Laden.

After 9/11, when coalition forces drove al Qaeda from their camps in Afghanistan, bin Laden and his cohorts reprised the proven tactic of recruiting foreign fighters. It was a way to get back in the fight. And the first great opportunity to go back on the offensive was Iraq.

Abu Musab al-Zarqawi slipped into Northern Iraq in 2002 to establish al Qaeda's new front well before the U.S.-led coalition launched its invasion. Openly proclaiming his allegiance to al Qaeda, Zarqawi put out the call for foreign fighters.

Those who answered his call executed countless bombings, kidnappings, armed assaults and beheadings. By the time of the Sunni awakening in 2005, the foreign fighters had started to wear out their welcome. Zarqawi met his end in 2006. But the pipeline tactic remains very much alive.

Wherever there is chaos and instability in the Islamic world, al Qaeda sees an opportunity to set up a foreign-fight pipeline to fuel the flames. Foreign fighters may help turn the tide of battle in their favor. But even if they don't, their presence can help link local insurgent groups to the greater cause of a holy war to build a global Caliphate.

Pipelines have been built into Yemen, Libya and Mali. The newest pipelines run from Lebanon, Jordan, Iraq and Turkey to "serve" Syria.

The Turkish pipeline delivers Islamist fighters from Western Europe to Jabhat al-Nusra and other fundamentalist groups fighting the Assad regime. More than 600 European Islamists are now fighting in Syria, half of them from France and Belgium.

Others hail from Denmark, Norway and Italy. Spain recently took down a terrorist cell whose primary purpose was to provide logistical support to al Qaeda's underground highway.

Al Qaeda will doubtless keep building pipelines wherever chaos presents an opportunity for "jihad." The U.S. and like-minded nations need to create a coordinated, systematic capacity to shut off these pipelines before al Qaeda turns them on. It takes a network to take down a network.

-James Jay Carafano, a Washington Examiner columnist, is Vice President for Defense and Foreign Policy Studies at The Heritage Foundation.