SOMETIMES WHAT WE DON'T KNOW can indeed hurt us. This was the case in 2006, when reporters noticed significant fighting between Iraqi insurgent factions. This confused journalists and government analysts, but the prevailing attitude was that if the insurgents were fighting each other, at least they weren't fighting us.

It turned out that the group that bore the brunt of this violence would later develop into the Anbar Salvation Front, which has proved to be one of our most important local allies in Iraq. Abu Musab al-Zarqawi, then the leader of al Qaeda in Iraq (AQI), was trying to wipe out this fledgling movement. If analysts better understood that situation, timely U.S. intervention could have thwarted Zarqawi and allowed the Anbar Salvation Front to make a difference on the ground sooner.

Similarly, today a critically important debate is raging about whether the United States should set a timeframe for troop withdrawal. While most people seemingly have an opinion on the matter, it's difficult to figure out whether the situation is truly futile without understanding the various factions that we're fighting.

Understanding the Iraqi insurgency is less difficult than most people imagine. A report that the International Crisis Group published last year concludes that the insurgency is "no longer a scattered, erratic, chaotic phenomenon," but that insurgent groups "are well organized, produce regular publications, react rapidly to political developments and appear surprisingly centralized." This article's goal is to paint an accurate picture of the insurgency as it exists today. The insurgency may look different six months or a year from now, but this is a critical time for understanding our foes, as the withdrawal debate reaches a fever pitch.

IN 2004 AND 2005, BAATHIST AND SUNNI nationalist insurgent groups comprised the bulk of the resistance movement in Iraq. These groups weren't necessarily waging a sectarian war, nor did they espouse a particularly radical religious creed. By late 2005, a number of secular and nationalist groups had decided to join the political process--which is traditionally how insurgencies are ended. Some Sunni insurgent groups even provided voters with protection against AQI during the December 2005 constitutional referendum. Alarmed, Zarqawi ordered the February 22, 2006, bombing of the Askariya mosque in Samarra. Askariya's importance to the Shia community was underscored by Iraqi vice president Adel Abdul Mahdi, who likened the mosque attack to 9/11.

This single bombing dramatically reshaped the entire insurgency. Shia reprisals were swift, devastating, and largely indiscriminate. These mass sectarian killings shattered the Baathist and nationalist insurgent factions. For rank-and-file Sunni insurgents, witnessing bloody attacks orchestrated by Shias made al Qaeda's sectarian arguments seem sensible for the first time. Today, the violence caused by the remaining nationalist groups is negligible compared to that caused by AQI: intelligence sources confirm that AQI and its ideological compatriot Ansar al-Sunnah are responsible for the vast majority of violence on the Sunni side. The most significant nationalist faction is the Islamic Army of Iraq--although even that ex-Baathist group now purports to have embraced a radical Islamic ideology.