Derek Harvey, who later served on Trump's National Security Council, told Reuters in 2015 that in 2003 and 2004 we had released assassins from the Badr Brigade who we had captured with target lists of Baathists and pilots because his superiors said that "this stuff had to play itself out". A May 28 2003 Manning cable on the rise of Islamic fundamentalism in Iraq noted Badr revenge killings at the beginning of the occupation and a strong public SCIRI/Badr presence.
The Badr Brigade was the armed wing of the Supreme Council for Islamic Revolution in Iraq (SCIRI). SCIRI and its Badr Brigade were a network of exiled Iraqis who had escaped to Iran during the Iran-Iraq war. Badr was trained by Iran and wanted, as SCIRI's name suggests, to replace Saddam with an Islamic theocracy up until the Americans invaded. According to this CFR page, Badr was only 10,000 strong in 2003.
Despite his early concerns over minor equipment coming from Syria and Iranian interference, rather than keep Badr exiled a couple more years, Rumsfeld allowed them into Iraq even though he publicly stated early on that they had been trained by the Iran's Islamic Revolutionary Guard Corps.
Why would he allow them to assassinate their own lists of Baathists? If this was counterinsurgency, why didn't we capture these targets for interrogation? After all, we thought that torture was so effective that it was necessary despite how it made us look. Why did we allow this in the first year of the insurgency? We had immediately fired 30,000 Baathists (mostly high-level civil servants) and 300,000-350,000 regular soldiers(on top of the more Saddam-loyal paramilitaries and intelligence services) who didn't fight us during our invasion and who had already taken all of their weapons from their bases. Yet the plan was to allow individual Baathists to be reviewed for evidence of crimes should they want to appeal.
What could explain all of this? Iraq had invaded Iran in 1980, but Iran turned the tables and by 1982 pushed into Iraq. We then secretly aided Saddam in a variety of ways so that Iran wouldn't know that we wouldn't let it win. More well-known is Iran-Contra. Half a million to a million people died because the Reagan Administration went to great lengths to keep our aid to Saddam secret. This is closer to our Iraq strategy than the "Salvador Option".
The Coalition captured a letter which they published in February 2004 from Zarqawi to Al Qaeda in which Zarqawi says that jihadis need to start a more open civil war against the Shia to mobilize Sunnis. Zarqawi cited only the Badr Brigade assassinations as a contemporary reason to bomb Shia, yet I have not found any news report mentioning this specific reference.
Before the Jan 2005 elections there was a Badr-linked sectarian special police force working for the provisional government known as the "Wolf Brigade". The CFR called the Wolf Brigade "one of Badr's recent offshoots" in June 2005.
After the elections Bayan Jabr, a Badr commander, was placed at the head of the Interior Ministry in May 2005. The Badr Brigade placed units in the national "special police commandos" who would go through checkpoints and kill Baathists systematically. Jerry Burke, an adviser to the Interior Ministry, told Musings on Iraq that the top brass prevented an attempt to surveil and stop these special police death squads.
Rumsfeld has released a "library" of documents which includes an email from James Steele that he forwarded to Cheney and Bush containing warnings that Badr police death squads could cause a sectarian civil war. He left only a cryptic comment and did nothing to prevent it.
After all of this and the Al-Askariyah (Golden Mosque) bombing, our military signed off on an agreement to integrate more than 1,000 Badr members as officers in the Iraqi army, as told by Badr leader Hadi al-Ameri when he complained that the US embassy was blocking the integration.
When we declared a new strategy, "the surge", after Rumsfeld was fired following the 2006 midterms, gun deaths immediately dropped in Baghdad. Total civilian casualties in Iraq were cut in half from July to September 2007, just before the 23% increase in troops was fully completed.
You might wonder why we kept troops there. Rumsfeld maintained that he wanted to leave as soon as we trained the security forces, even before we quelled violence.
The administration claimed that Iranian proxies were killing our troops as early as 2004, and Rumsfeld even suggested that this might be happening as early as June 30, 2003, just three months in. Yet inexplicably the administration showed no concern that the Badr Brigade, obviously Iran's proxy if there ever was one, might use their free reign to give insurgents weapons, money, intel on VIP or troop movements (to attack them and/or notify insurgents that it wasn't safe to transport weapons), or uniforms to access secure areas and kill VIPs. This means that either the administration didn't believe that Iranians wanted to kill Americans, or they just didn't care about any deaths Iran could cause. Considering the vulnerability of VIPs, I conclude that the former is the case.
Given all of this information, our military should prove the allegations from the Bush period before we believe them. They should release the recordings of purported confessions by Iranian-trained militants that have been paraphrased in text, along with all of their info on the confessors to independent (perhaps European) governments and human rights groups so that they may be reviewed for authenticity. If that happens, we will know whether Iran trained anyone to plant IEDs. If we can't see the recordings, we have to presume that the texts are more falsified evidence from the neocons heading the military.