All battles and bombardments share their secrets one by one. Eastern Ghouta is no different. Why the sudden, savage bombardment of these Syrian towns and villages more than three weeks ago? Why the wasteland of homes and streets – and how did so many of the civilians survive, along with hundreds of Islamist gunmen?

You can do no better that start your enquiry in a frontline dugout near Arbeen, on the old and now war-smashed international highway between Damascus and Aleppo. It is protected by oil barrels of solid concrete, an iron roof, a rocket-propelled grenade launcher, a couple of rifles and a rusting motorbike, presumably to carry messages when the lines were cut. “Twenty mortars a day,” one of the Syrian soldiers says, rolling his eyes.

And now it is over, he hopes. But – aside from the oil lamps and the cups of ‘mutta’ tea (an outrageous non-alcoholic brew originating from Argentine plants, which the Syrian army drinks by the pint) – what catches your attention is the absence of a single trench.

The soldiers sport beards like the French ‘poilu’ in the Great War a hundred years ago. But they dig no trenches. Not a single communications alleyway winds through the dirt and mud on either side of the dugout to give the running messenger cover from those mortars. Maybe the motorbike increased their chances. But no one has ever provided me with a serious explanation of why the lattice of frontline trenches and rear trenches and revetments dug so deeply a century ago – deeper by the Brits than by the French, deeper still by the Germans – has never caught on in Syria.

Thus many of these Syrian soldiers were shocked to find how safe their enemies were. Here is the account of an eyewitness who entered the ruins after the first Syrian assault units burst through the front lines towards Douma. “I have never seen so many tunnels. They had built tunnels everywhere. They were deep and they ran beneath shops and mosques and hospitals and homes and apartment blocks and roads and fields. I went into one with full electric lighting, the lamps strung out for hundreds of yards. I walked half a mile through it. They were safe there. So were the civilians who hid in the same tunnels.”

Buses entered the last rebel holdout of Syria’s eastern Ghouta on 3 April in preparation for a possible second day of evacuations (AFP/Getty) (AFP/Getty Images)

These great stoneworks – for they were carved through the living rock, supposedly by Palestinians on loan from Hamas, men who had spent their years hacking tunnels between Gaza and the Egyptian desert to the south – have become a familiar part of the Syrian war. I have walked through them in Homs, where the makers carved their names on the walls like Victorian railway builders, and in eastern Aleppo. These tunnels somehow carry inside them the necrology of ideas, the ideological martyrs’ cemetery of their makers’ minds. They are deep and dank and glisten with moisture. But they are safe.

So here comes the latest little secret of the Ghouta war. The Syrian aircraft – so often blamed for the indiscriminate nature of a bombing campaign which, according to many reports, has killed 1,500 civilians in eastern Ghouta – were old. But the Russian aircraft were also old Sukhoi 24s, some of them upgraded but others inferior to the Sukhois currently sold by Moscow to the government of Belarus. And this from a Russian source – outside Syria, but all too familiar with Russian military operations inside the country – who knows about the trajectory of rockets: “The bombs we used in Ghouta were not ‘smart’ bombs with full computer guidance. Maybe some. But most had a variable of 50 metres off target.” In other words, you can forget the old claim of “pinpoint” accuracy which Western armies also like to adopt. These Russian bombs launched against eastern Ghouta had a spread pattern of 150 feet each side of what the pilots were aiming at; which means a house instead of an anti-aircraft gun. Or one house rather than another house. And anyone inside.

Faces from the Siege: Eastern Ghouta Show all 10 1 /10 Faces from the Siege: Eastern Ghouta Faces from the Siege: Eastern Ghouta Newly arrived civilians from el-Ghouta packed onto a government refugee bus on the front line outside Damascus Nelofer Pazira Faces from the Siege: Eastern Ghouta A small Syrian refugee girl ponders her future on the bus taking her from the eastern Ghouta front line Nelofer Pazira Faces from the Siege: Eastern Ghouta A refugee woman from el-Ghouta who lost her husband and son in the early bombardment by the Syrian army Nelofer Pazira Faces from the Siege: Eastern Ghouta Civilian refugees less than an hour after crossing the front line from their pulverised homes in el-Ghouta Nelofer Pazira Faces from the Siege: Eastern Ghouta Happy to be seen, anxious to conceal their faces: Syrian men who have crossed the east Ghouta front line waiting to be interviewed by Syrian security officers in a nearby refugee camp Nelofer Pazira Faces from the Siege: Eastern Ghouta 29-year old Ala'a Younis, paralysed after a neck wound in eastern Ghouta sits on his wheelchair beside his father Mohamed at a refugee camp near the front line Nelofer Pazira Faces from the Siege: Eastern Ghouta A wheelchair-bound refugee from el-Ghouta describes how he was forced to live in tunnels along with families and Islamic fighters Nelofer Pazira Faces from the Siege: Eastern Ghouta This young woman lost three members of her family in eastern Ghouta before the end of the siege Nelofer Pazira Faces from the Siege: Eastern Ghouta Sana el-Boukeri in a Syrian refugee camp. She lost her civilian husband and son in the Syrian and Russian bombardment of eastern Ghouta while her other son was wounded fighting in the Syrian government army on the other side of the front line. Nelofer Pazira Faces from the Siege: Eastern Ghouta Both sides of history. Sana el-Boukeri's husband and son were killed in the Ghouta bombing by Syria, her other son wounded in the government army Nelofer Pazira

But these blockbusters, it seems, couldn’t bust any blocks. The tunnels were never breached. That’s why they were built. They were bomb-proof. And thus the Russians and Syrians fired more and more bombs to break them. The Islamist groups in Ghouta did not have barracks or dugouts – not in the traditional sense, at least – for they lived in the tunnels, ate in the tunnels, fought, just briefly, in the daylight outside the tunnels, and then dragged their mortars back inside. A fighter wishes to pray: he can take the tunnel to the mosque. He needs surgery? He can be taken between those glistening walls to the hospital. He needs to move to a new battlefront, he takes a mile or two walk across town. Underground.

When the Syrian forces of the ‘Nimr’ units – those soldiers commanded by Brigadier General Suheil al-Hassan, ‘The Tiger’ – advanced, they came across civilians with their hands up. A man who watched the helmet camera videotapes of these soldiers spoke to me of what he saw. “One man came out and they shouted at him to stop – and to raise his shirt to show he didn’t have a suicide bomb. But he didn’t stop. He kept on walking. They shouted again and started firing at the ground and the walls around him. Then he stopped and understood and raised his shirt. A woman came out of one house with her hands up, but the soldiers were then ambushed by armed men and shouted at her to return to her basement.”

The Syrians paid for their advance. In one short battle, at least 20 of them were killed. During another, five men emerged from the ruins, all dressed in Syrian army uniform and carrying weapons, well shaved, saying they were “coming across” to the Syrian lines. Several Syrian soldiers captured years ago were still held hostage by Islamists in underground cells. But the ‘Nimr’ units, while they knew the uniforms were real, looked closely at the faces of the men wearing them. “They could see they were newly shaved, that they weren’t so tanned on their chins as on the rest of their faces, and they realised they were Jaish al-Islam men dressed in Syrian uniform who had just removed their beards,” the eyewitness added bleakly. “They killed them all.”

So why the ferocity of the bombardment? That Russian source – no politician but certainly, I suspect, a Putin supporter – believes to this day that the Russian president wanted to end the Syrian war, especially the Ghouta conflict, before his election. But this proved impossible. Syria doesn’t fit the familiar ‘quagmire’ of Vietnam legend; it is a vast terrain of captured and recaptured and re-recaptured towns and villages, which moves with the power of the antagonists. The Russians can pick and choose their battles. This increases mobility. But it doesn’t create the exit home.

There are streets in Ghouta, incredibly enough, whose buildings are still standing relatively unscathed. They were spared during the bombardment because their inhabitants said – by mobile phone -- they wanted to stay in their homes and would not resist the Syrian army. Thousands of Syrians in Ghouta have thus not joined the refugee buses nor – even if they were related to the Islamists – accompanied the women and children travelling with their jihadi menfolk to Idlib. They are still living at home.