While the Ukrainian city was under siege by Russian forces, a team of journalists recorded the brutal war, resulting in this essential documentary.Everyone else was gone: the authorities, the aid workers, the other journalists too. One week into Russia’s full-scale invasion of Ukraine in February 2022, the Associated Press video journalist Mstyslav Chernov was still in the port city of Mariupol, watching from a high floor of a hospital as a tank emblazoned with a white Z pulled up alongside. Smoke kept rising, bitter and black, from the shelled housing blocks a short distance away. There was no way out. Mariupol was surrounded now. Chernov kept his cameras rolling.“20 Days in Mariupol,” a relentless and truly important documentary, engulfs us in the initial ferocity of Russia’s siege of a city whose name has become a byword for this war’s inhumanity: My Lai, Srebrenica, Aleppo, Mariupol. The A.P. journalists were the last from an international news organization in the city, and for three weeks they documented pregnant women fleeing a bombed maternity hospital, the elderly and the displaced boiling snow to obtain fresh water, the freshly dug ditches where children’s corpses were laid to rest. The reporting would win Chernov, along with his colleagues Evgeniy Maloletka, Vasilisa Stepanenko and Lori Hinnant, this year’s Pulitzer Prize for public service, but because internet connections were sparse to absent in the city, Chernov could only transmit a small fraction of his footage during the siege. It all comes out in “20 Days in Mariupol,” in which the battle to survive in southeast Ukraine becomes entwined with the struggle to tell the world what’s happening.This film is very hard to watch, and so it should be, though its episodic structure makes it somewhat easier to endure: Day 1 through Day 20, one at a time, from the first bombs to the team’s flight to safety. On the morning of Feb. 24, Chernov and his colleagues head toward Mariupol, a city of half a million people on the Sea of Azov, and drive by Ukrainian military bases whose antiaircraft systems are burning — the first Russian targets, to prepare the path of their war planes. Many residents doubted the violence would reach Mariupol, and evacuation trains were leaving the city half-empty. Now we follow them into improvised shelters: a cold cellar, a CrossFit gym. “I don’t want to die,” says one young boy. “I wish it would all end soon.”But by Day 4 the fighter jets are overhead, and Chernov is stationed at one of Mariupol’s remaining open hospitals, about a mile from the front line on the edge of the city. He’s there when an ambulance rushes up, and paramedics perform C.P.R. on a 4-year-old girl named Evangelina, severely injured after a Russian shell landed near her home. The medics race her to the modest emergency room, where her blood pools on the floor as they try, and fail, to resuscitate her. (Chernov blurs out her face here, though The A.P. published uncensored images at the time.) “Keep filming,” the head doctor insists — and a minute later, we see the same footage of the doctors at work in grainy reproduction on an MSNBC broadcast and Britain’s ITV News.This blending of high- and low-resolution video registers is a critical tool of Chernov and his editor, Michelle Mizner of “Frontline,” who in many chapters of “20 Days in Mariupol” suture together three kinds of imagery. First comes drone footage of the city — its Khrushchev-era housing blocks, its huge Azovstal steel plant — whose devastation becomes more visible as winter passes to spring. Then follows unique documentation of the war’s early atrocities, shot on high-definition video, but often askew or rocky as Chernov runs after a hospital gurney or flees from the aim of snipers. Finally, at the end of many days, the footage repeats as broadcasts on CBS News, France 24, Deutsche Welle and other AP clients.Even if they feel a touch self-congratulatory, these rebroadcasts underscore two things: the rarity of Chernov’s footage, and the immense challenge of getting it out of Mariupol. The port city’s internet is basically gone by Day 11, when the Russians blockade it from all three sides, and the A.P. journalists risk their lives to hunt for wireless connections after curfew. And there is the matter of Chernov’s nationality. Though he has covered wars in the Middle East and Southeast Asia, he is a native of Kharkiv, Ukraine, and as he whispers at the start of the film, “I have no illusions about what will happen to us if we are caught.”Day 20 comes, and with it a harrowing drive past a dozen Russian checkpoints, with the journalists hiding cameras and hard drives under the seats. One day later, on March 16, 2022, Russian forces bomb the city’s Drama Theater, where hundreds of adults and children have taken shelter. This documentary is more, therefore, than a unique record of particular crimes; it’s a synecdoche for a much larger atrocity, and a model of how we discover the larger truth of war in images of one hospital, one grave, one child.“With every new war, the ethics of war photography are debated again,” regretted the Ukrainian art historian Kateryna Iakovlenko in a recent essay on our self-serving doubt of depictions of horror, made acuter through Russia’s parallel disinformation campaigns. The only moral question before us is whether we take these images seriously, or whether, with a skepticism also known as cowardice, we turn away.20 Days in MariupolNot Rated. Running time: 1 hour 35 minutes. In theaters. More