"It was during the decade of sanctions, as central power weakened, that the fundamentalism and Salafism associated with the insurgency grew roots in Iraq. The works of the Iraqi exile thinker Muhammad Ahmad al-Rashid on jihad and the Islamic state, which were smuggled from Egypt, gained a following, but more important, the tentacles of fundamentalist and Salafist trends popular elsewhere in the region began to penetrate the country.
The Muslim Brotherhood, with its strong presence in Jordan and Syria and affiliation with the Wahhabi establishment in Saudi Arabia, established itself in the Sunni regions of western Iraq that border on Jordan and Syria, and is today entrenched in the Iraqi Islamic Party. Firebrand preachers such as the Egyptian Hamid Kashk and the Syrian Mahmoud Qoul Aghassi (also known as Abu Qaqaa) were popular with Iraqi Sunnis even before the war, and copies of their sermons became ubiquitous in Sunni towns afterward. Throughout the decade of anti-Saddam sanctions, Jordanian mosques remained concerned with the plight of ordinary Iraqis and raised funds that were disbursed through mosque networks in Iraq. Also active in Iraq were Islamic activists such as Laith Shubailath of Jordan and extremist Salafist activists – some with close ties to al-Qaeda – such as Muhammad al-Maqdisi, who is a friend and by some accounts one of Zarqawi's mentors. Before the war toppled the Saddam regime, extremists from everywhere would come to the Jordanian-Iraqi border towns to buy weapons smuggled out of Iraq. Trade in weapons created ties that straddled boundaries and gave extremists contacts and allies inside Iraq, and in turn made it possible for the growing insurgency in that country to spread across the region.
Extremist activists had an incentive to move into Iraq. They were losing ground to the more moderate main body of the Muslim Brotherhood in Jordan and were often hounded by Jordanian or Syrian security forces. Their activism over the decade leading to the Iraq war not only introduced fundamentalist thinking in al-Anbar but also created organizational links that would facilitate the insurgency after the war. In fact, Zarqawi's emergence as a force so soon after the war reveals the extent of involvement of Jordanian Salafis in Iraq. Extremist ties between Jordan and Iraq ran in both directions. The flight of hundreds of thousands of Iraqi exiles and refugees to Jordan and Syria has only expanded the opportunity for building extremist networks that cut across national boundaries. The extremism that was exported to Iraq before and during the war, strengthened and radicalized, spread back into Jordan. The same ties also made developments in Iraq directly relevant to Islamic activism in Syria and Saudi Arabia."
–Vali Nasr, The Shia Revival