Iraqi Memories of 1991
In understanding the development of today's sectarian turmoil in Iraq, one good place to start is the March 1991 uprising against Saddam Hussein in the aftermath of Operation Desert Storm. A complete history of this has yet to be written, though there is an important oral history project underway. What I know of these matters comes primarily from books by Eric Davis, Fanar Haddad, and Dina Rizk Khoury.
The revolt had nothing to do with religious ideology, except probably in terms of fortifying many people to take on the struggle spiritually and providing a defiant iconography against a regime that had suppressed Shi'ite rituals and centers of authority. It was, if anything, a revolt against the cost for individuals of Iraq's military failures. It began among troops returning from the front against the U.S.-led coalition, and was most intense in areas that had been hit hard not only during Desert Storm, but the Iran-Iraq War of the 1980s, where the most intense fighting took place in the south.
Although there were incidents in most Iraqi provinces, it was overwhelmingly a southern revolution. A conventional explanation is that the use of Shi'ite symbolism that put off Sunnis, though I find myself wondering if the motives to revolt were also distributed unevenly geographically. In any case, it became a Shi'ite uprising, especially in historical memory, and the divided historical memories of it became an important fissure going forward. After the revolt was suppressed, a series of articles appeared in a leading government media outlet which presented an interpretation of events that Sunni Arabs, who had no other information and probably had some predisposition to the cultural stereotypes on which it drew, largely accepted. The articles, which Saddam Hussein himself may have written, claimed the revolt had been instigated by large numbers of Iranian instigators and was largely carried out by uneducated people from the social margins - marsh dwellers and the Shi'ite urban poor. Indeed, "Mob's Rebellion" came to be one term used for the uprising among Arab Sunnis.
During the Iran-Iraq War, the Iraqi internal self-presentation was that they were fighting for a modern secular lifestyle against a medieval theocracy led by clerics whose followers obeyed them blindly. I find myself wondering how much the Ba'athist portrayal of 1991 echoed some of that propaganda. The bottom line is that for Sunnis, then, the revolt was ultimately against what they understood Iraq to be, along both class and ethno-religious dimensions. For Shi'ites, however, they were the Iraqi nation rising against a despised dictatorship, and they did not understand how Sunnis would not join them, and instead appeared to cheer on their defeat.
These conflicting narratives continued to fester during the 1990's, and serve as some of the background for why parties acted as they did after Saddam's ouster by the 2003 Anglo-American invasion. In the wake of that, of course, Sunnis still saw foreign hands as behind the Shi'ite rise and themselves as the guardians of the nation, while Shi'ites saw themselves as the nation liberated having to fight off remnants of the old regime who were little more than terrorists allied with international salafi jihadists.
In other words, while Sunni and Shi'ite are opposed in modern Iraq, there is nothing timeless and unchanging about that, nor is it because of centuries-old religious disputes or theological differences. If you went to Iraq and asked people of one community about the other, you might hear some cultural stereotypes that may or may not be tied to religious beliefs and practices, but the most emotional issues would be grievances concerning what has happened in the last generation, who can be trusted to defend a particular view of Iraq as opposed to betraying it to outsiders, and what their perceived plans are for the future.