On February 22, 2012, when the British photojournalist Paul Conroy survived the artillery barrage that killed Marie Colvin, he was rushed to a place of greater danger. Bashar al-Assad’s war of repression has killed civilians indiscriminately, but its targeting of medical facilities has been systematic. Hospitals are the most endangered spaces in opposition-held areas. Of the 492 medical facilities destroyed in the war, Physicians for Human Rights attributes the destruction of 446 to Assad and his allies. The UN Commission of Inquiry has charged the regime and its allies with having “systematically targeted medical facilities… and intentionally attacking medical personnel.” With a pierced abdomen and a fist-sized hole in his thigh, Conroy was carried to hospital under a hail of mortar fire. It was the only hospital in Baba Amr, the besieged Homs neighborhood Colvin and Conroy had been reporting from—and it had no anaesthetics. As the hospital’s only doctor cut away Conroy’s torn muscles and stapled his wounds, Conroy had to dull the pain with three cigarettes.

Conroy was where he wanted to be, but not in the manner he had intended. A day earlier, he had convinced Marie Colvin, the intrepid Sunday Times correspondent, that the situation in Baba Amr was too dangerous for them to stay. Colvin had agreed to leave on the condition that they visit the beleaguered hospital one more time. The day before that, Colvin had also made the fateful decision to speak to the BBC and CNN about the dire situation inside the siege. She was aware that the broadcast would reveal her presence to the regime, putting her life in danger. A Lebanese intelligence officer had earlier warned them both that regime troops had orders to execute any Western journalists on the spot. (New information suggests that Colvin was indeed actively targeted by the regime.)

The regime had failed to thwart their entry, but it was determined to prevent their exit. Early the morning after her last broadcast, the regime started its assault on the activist-run media center where Conroy and Colvin were housed. A former artillery gunner in the British Army, Conroy quickly judged that the barrage was targeted at the media center. But before he could warn Colvin, the center had taken a direct hit, killing Colvin and the French photojournalist Rémi Ochlik, wounding Conroy and others.

The incident was a turning point. It signaled the regime’s willingness to use deadly violence to thwart independent witness to its slaughter. Baba Amr would be the last time that the foreign press was able to infiltrate a siege and refute the regime’s propaganda claims. By eliminating independent reporting, the regime would successfully turn subsequent atrocities into cases of “he said, she said”—the regime’s word against the opposition’s, with audiences inclined to accept or reject them based on their political predilections. The regime’s (and later Russia’s) use of the “war on terror” tropes would resonate well with Western audiences, especially policy-makers and think-tankers. All Assad’s opponents would be declared “terrorists.”

Colvin and Conroy’s journey into Homs and the latter’s near-miraculous escape are the subject of Chris Martin’s gripping new documentary Under the Wire. The documentary is based on Conroy’s book of the same title, which is itself an extraordinary work of, and about, journalism. Martin brings the story to life with dramatic reconstructions, archival footage, and eyewitness testimonies. The film features interviews with many of Colvin’s companions and colleagues, but the most compelling testimony remains Conroy’s own. His recollections give the film its poignancy. (Since his recovery, Conroy has remained involved in Syria and, in partnership with the medical charity CanDo, has delivered medical aid and helped set up hospitals in areas under regime assault.)

Colvin and Conroy had entered Baba Amr on February 15, 2012, after a perilous journey across the Lebanese border and through the Syrian countryside. To enter the besieged zone, they had to crawl through three miles of a drainage pipe whose exits were exposed to regime snipers. The duo had seen more wars between them than do most soldiers, and they could tell immediately that this one was different. “This wasn’t war,” says Conroy. “It was slaughter.” With help from local rebels, and pursued by sniper, rocket, and artillery fire, Colvin and Conroy eventually made it to the local media center, “the headquarters of a hunted and starving band of outlaws.” The house next door had already been destroyed in a direct artillery strike, killing four women. Not much farther was the “widow’s basement,” an underground space where they found nearly 150 women and children living in terror, with little food and even less hope. At the local hospital, they met Dr. Mohamed al-Mohamed, a physician overwhelmed by a surfeit of death and mutilation, with corpses strewn around the operating room without even blankets to cover them. They witnessed the agony of a dying three-year-old. “We have to show this to the world,” said Colvin. “We will make a difference.”

It was this faith in the power of journalism that compelled the pair to return to the front line after they had safely got out of Baba Amr on February 17. Expecting a regime offensive imminently, rebels and activists had forced them to leave; but when the attack didn’t materialize, Colvin felt a responsibility to return. Conroy had misgivings but was motivated by the same impulses. They took the drainage pipe back to Baba Amr. At the media center, the video of another dying child only steeled their resolve. The attacks on the neighborhood had meanwhile intensified.

Colvin was killed hours before she could visit the hospital whose horrors she had wanted to convey fully. Conroy was wounded and trapped: the drainage pipe had been blown up and the regime had ordered a manhunt for him and other surviving journalists. At one point, the regime sent an ambulance with the promise of a safe exit, but the doctor who came with it quietly warned Conroy against getting into the ambulance: he would never have made it out alive. In the end, it took a heroic effort by activists and rebels to smuggle Conroy out of Baba Amr, many losing their lives in the process. When Conroy balked at the prospect of his being saved instead of wounded children, one activist told him: “We want you out alive to tell our story. Please, go!” Like Colvin, they also believed in the power of journalism.

Journalism, however, proved inadequate to the task. Knowledge, it emerged, is not power; and power, when left unchallenged, can bend knowledge to its will. Doubt became a weapon: with politicians in the West reluctant to act, it kept public opinion conveniently inert. The Assad regime invested much effort in manufacturing uncertainty; and though the absence of independent witnesses did not make verification impossible, it certainly made it more onerous and time-consuming. By the time the truth of an atrocity could be established, news headlines had moved on. The regime suffered no consequences for killing or impeding journalists, while controlling their access had long-term advantages. The Assad government issued visas selectively and its minders herded journalists to stories that were consonant with its narrative. It rewarded deference and punished criticism. The journalist (and Review contributor) Janine di Giovanni, for example, was blacklisted for reporting critically on its repressive rule; the Financial Times’s Erika Solomon or Vice’s Isobel Yeung will likely never go back; but others, such as Robert Fisk, Patrick Cockburn, and Charles Glass, can frequent Damascus unimpeded.

A striking feature of Under the Wire is that it captures the reality of Syria before this degradation. Earlier in 2012, journalists were welcomed in Syria as truth-tellers and potential agents of change who could help amplify Syrian voices and prompt humanitarian assistance. Journalists like Colvin and Conroy in turn accepted the responsibility, bore witness, and tried to shake the West’s inertia. They risked their lives to be in Baba Amr and Syrians risked theirs to facilitate their work (their translator, Wa’el al-Omar, declined payment, accompanied them into danger, and refused to abandon them when he was wounded). Colvin’s killing and the impunity of her killers would wreak havoc on these expectations. The consequences would be fatal—for Syrians foremost, but for the rest of the world as well.

Shortly after the assault on Baba Amr, the regime carried out its first major massacre of civilians, in Houla on May 25; then again, in Qubeir on June 6; and in Daraya on August 20; and again, in Baniyas and Bayda in early May 2013. And so it went. The regime also switched to the indiscriminate use of airpower. Meeting no resistance to its strategy of total war from the international community, it finally tested Barack Obama’s “red line” with a chemical attack on Eastern Ghouta on August 21, 2013, that killed over 1,400 civilians. Obama’s retreat from action against the use of chemical weapons signaled to Syrians that the world was indifferent to their suffering; it signaled to the regime that there was nothing it couldn’t get away with. That sealed Syria’s fate.

This also had consequences for journalism. If reporting evidence of crimes brought no consequences for the criminals, then truth had lost its value and journalists had become dispensable. Few Syrians would again risk their lives to help a journalist. In early 2012, Under the Wire shows Dr. Al-Mohamed welcoming journalists to his overwhelmed hospital, appealing through them to the world. A few months later, in Aleppo, a doctor would refuse to speak to the American journalist Clare Gillis, seeing reporters as nothing more than voyeurs. The cynicism about the Western media only hardened after Obama’s breached red line.

By actively targeting media staff, the regime also made it too dangerous for journalists to visit opposition-held areas. This has allowed it to conduct its subsequent rampages under a pall of uncertainty, assisted by its Western apologists. A view of the conflict has prevailed where people confine themselves to generalized condemnations of violence, leery of assigning blame. These “blame on both sides” refrains seem obscene in light of the mass of evidence painstakingly gathered over the war’s course by journalists, war crimes investigators, human rights groups, medical professionals, academic researchers, and intelligence agencies. All point to one incontrovertible fact: that the regime and its allies are the main authors of violence in the conflict. In the words of the UN Commission of Inquiry, the regime is responsible for “the crimes against humanity of extermination; murder; rape or other forms of sexual violence; torture; imprisonment; enforced disappearance and other inhuman acts.” Yet, even as the regime begins its assault on Idlib, the European Union is debating formulas for rewarding Assad’s government with reconstruction money, eager to hasten the restoration of “stability” so that it can begin repatriating Syrian refugees.

Was Colvin’s sacrifice worthwhile? It is tempting to say no—that in an indifferent world, no story is worth dying for. But Colvin’s life is itself a rebuke to such pragmatism: she had risked her life before and saved many other lives as a result. In 1999, in East Timor, for example, Colvin stayed with 1,500 besieged civilians under fire, even as twenty-two other journalists left, continuing to raise awareness until the coverage embarrassed authorities into evacuating all the civilians. Colvin was neither naïve, nor cynical. She certainly did not have a death wish: in her characteristic wry style, she mocked the idea of dying for stories. “Kinda defeats the purpose,” she said. In Syria, it wasn’t her expectation that was unreasonable; it was the world’s response that proved woeful.

Five days before her death, she had already left Baba Amr once; and she could have safely gone home. But she returned as an act of solidarity. As in East Timor, she hoped that her presence would stir the world’s conscience. Colvin could not be indifferent. Under the Wire is a fitting tribute to Colvin’s extraordinary life. It is not just a superb film; it is also a testament to the value of journalism and a warning to a world that fails to protect its truth-tellers.

Under the Wire will be in theaters in the UK from September 7. It will also be broadcast as part of the BBC’s Storyville series, and on the History Channel in the US. It will also be available on iTunes from September 7.