Prosecutors plan to call dozens of Iraqis to testify over bloodshed in Baghdad that inflamed anti-US sentiment around the world

After years of delays, four former guards from the security firm Blackwater Worldwide are facing trial in the killings of 14 Iraqi civilians and the wounding of 18 others in bloodshed that inflamed anti-American sentiment around the globe.

Whether the shootings were self-defense or an unprovoked attack, the carnage of 16 September 2007 was seen by critics of the George W Bush administration as an illustration of a war gone horribly wrong.

A trial in the nearly seven-year-old case is scheduled to begin with jury selection on Wednesday, barring last-minute legal developments. Prosecutors plan to call dozens of Iraqis to testify in what the Justice Department says is likely to be the largest group of foreign witnesses ever to travel to the US to participate in a criminal trial.

The violence at the Nisoor Square traffic circle in downtown Baghdad was the darkest episode of contractor violence during the war in Iraq, becoming one more diplomatic disaster in a war that had many. Iraqi officials, who wanted the guards tried in a local court, were outraged.

In the trial, defense lawyers will focus on the guards' state of mind in a city that was a battleground.

Car bombs and insurgents were daily perils for the Blackwater teams. As part of its work with the State Department, Blackwater had a team of 15 intelligence analysts who produced daily threat updates, colored maps of a city riddled with bomb blasts.

"The core disputed issue in this prosecution is self-defense – whether the defendants believed that deadly force was necessary to defend themselves and their teammates from an insurgent attack, and whether that belief was objectively reasonable," lawyers for the guards said in court filings.

Reasonableness shouldn't be judged on "hindsight analysis in a courtroom seven years and thousands of miles removed from the event", the lawyers argue in the filings.

In the aftermath of the shootings, Blackwater Worldwide provided the government with photos of the guards' vehicles pocked and streaked with bullet marks.

On Capitol Hill, one theme in politically tinged congressional hearings was that hiring large numbers of security guards, and letting them operate outside the military chain of command in a war zone, was a recipe for disaster.

Blackwater founder Erik Prince declared: "I believe we acted appropriately at all times." The Nisoor Square shootings spelled the death knell for his company. Formerly known as Blackwater Worldwide, the company is under new ownership and Prince is no longer affiliated with it. The company was sold to a group of investors who changed the name to Academi.

The very presence of security guards in Iraq is touchy. Lawyers for the guards had asked that current or former members of the military not wear uniforms when they testify in the case, a request the court denied. The guards' lawyers argued that distinctions between contractors and uniformed military had become highly politicized.

In 2009, US district judge Ricardo Urbina dismissed the case against the Blackwater guards. From the Iraqi government's perspective, the dismissal was an example of Americans acting above the law. Urbina said government lawyers ignored the advice of senior Justice Department officials by building the criminal case on sworn statements of the guards given under a grant of immunity – meaning the guards' own statements could not be used against them.

Two years later, a three-judge panel of the US court of appeals for the District of Columbia circuit revived the prosecution, ruling that Urbina had wrongly interpreted the law. The decision gave the Justice Department another chance.

In the upcoming trial, one of the guards, Nicholas Slatten, is charged with first-degree murder. The other three guards – Paul Slough, Evan Liberty and Dustin Heard – are charged with voluntary manslaughter, attempted manslaughter and gun charges. Slatten could be sentenced to life in prison if convicted, while the other guards face a mandatory minimum penalty of 30 years in prison if a defendant is convicted of the gun charge and at least one other charge.