The veteran BBC reporter has been on the ground in Syria since the early days of hope in the Arab spring. It’s her expert witness that brings so much weight to this two-parter

The worst humanitarian crisis of the century. A conflict that has gone on longer than the second world war, drawing in 75 countries and counting. Half a million killed. Millions displaced. A country in utter ruins. And still, seven years on, no military solution, no prospect of a diplomatic answer and no end in sight. This tremendous – and necessarily distressing – documentary (part two is on Friday), fronted by the veteran correspondent Lyse Doucet, begins with the now stock phrases and statistics that trick us into thinking we know this war. Then it tells the story of what actually happened. The facts, as they used to be known.

And we need to be reminded. The appalling truth of a war so long and entangled in world politics is that you become confused, disengaged and desensitised. Despair blots out the need to know and keep knowing. This is how we begin to forget why wars started in the first place.

Doucet, who has been on the ground in Syria since the beginning of the conflict and is a reminder of the courage and tenacity of old-school reporters, begins with the peaceful protests in Homs in May 2011. She interviews Noura al-Jizawi, a student who, emboldened by the Arab spring like thousands of others, risked everything to take to the streets against the Assad regime. “It was an amazing day,” Noura recalls, describing it as the first time in her life she had a voice. When the protests swept the country and Assad responded with a brutal crackdown, Noura became a daughter of the revolution. She was kidnapped and detained in the notorious Sadynaya prison. Images taken inside by a government photographer show numbered and recorded dead bodies. What more damning evidence of state-sponsored torture is there?

And yet. “We detain terrorists,” the Syrian foreign minister Faisal Mekdad maintains when Doucet asks him about the scores of imprisoned and disappeared doctors, teachers and students. “We detain potential violators of law … We don’t detain citizens at large.” Time and time again, key players on all sides engage in doublespeak or refuse to own their part. The overwhelming sense is of opportunities missed and gross inaction. One question posed by a documentary that is so nuanced it left me more confused than when I started (which is a compliment, by the way) is: “How did the world come to abandon Syria?”

For Robert Ford, the US ambassador to Syria during the Obama administration, the president initially calling on Assad to resign was a mistake that created expectations: “I should have objected when I was called the night before.” For others in the inner circle, his reluctance to intervene made sense in an insane situation. “President Obama would always ask: ‘How is that actually going to fix the problem?’” says the former senior foreign policy adviser Derek Chollet of the CIA plan to arm rebel militias. “Then everyone looked at their shoes and said: ‘It won’t.’” Obama’s response, he added, was invariably: “We’re not doing this just to make ourselves feel good.” Which is not something you can imagine the incumbent US president saying.

While state department officials placed bets that Assad would be gone by Thanksgiving 2012, Marwa al Sabouni, an architect who stayed on in besieged Homs with her children, recalls her son blocking his ears at the sound of mortar rounds landing. Muhammed Jinnyat, who defected to join the Free Syrian Army, explains how starving civilians trapped in rebel-held areas resorted to eating leaves from trees. The trees became “bait” and snipers were positioned near them. “Many people died this way.”

In deeply distressing footage from 2012, Doucet reports on families massacred in their own homes by armed gangs who villagers claim were from the Alawite sect, fiercely loyal to the Assad regime. “A woman. Completely charred. In her own bed,” says Doucet, her words whittled down by horror as she stumbles out of a house.

Perhaps most upsetting of all, especially less than a month after Douma, is photographer Artino’s account of filming the aftermath of the August 2013 chemical attack in Damascus. “I see them as children sleeping,” he says of the babies and children he photographed on the ground. “Their eyes are closed and they are wearing pyjamas.” When he realised they were not sleeping the camera fell out of his hand and he went down, poisoned by the sarin gas that killed 1,000 people and was attributed to Assad. Later, Artino and his friends borrowed a generator and clubbed together to buy a TV so they could watch how President Obama, and the world, would react. “We were like, no way. Did he just say he’s not going to do anything?” Artino says. “We felt hopeless. We felt left behind.”