'We are used to death,’ said Ismail. He had been to the funerals of four friends in a single week, all killed by aerial bombs. ‘We’re used to bloodshed. We’re adapted to the situation and this style of life now. It’s normal. If you lose someone, then the next day you say, OK, life must go on.’

Ismail spoke to me from eastern-Aleppo, where as many as 250,000 people are under siege by the Syrian regime and ‘living on rice’, as he described it. He is in his late twenties and is one of the White Helmets, the civil defence volunteers who dig people out of the rubble after an attack. He could not endure the despair on the faces of the injured who knew they would not survive, he said.

In just two weeks, since a ceasefire ended in a ‘rain of bombs’, the White Helmets have documented some 400 civilians killed in Aleppo, more than a third of them children. Their work is extremely dangerous because of the regime’s tactic of sending the planes in again after rescuers arrive. ‘Double tap: it’s famous now,’ Ismail said. ‘All the guys we lost [were] in that scenario… They [the regime] do not discriminate. They kill everyone; they don’t care… They are monsters.’

The bombing has escalated now, because the regime has begun an offensive to retake Aleppo, the last city the armed rebels hold anywhere in the country. One account says that so many incendiary bombs are being dropped that a child woke her mother in the middle of the night to ask if it was morning. Syria’s Guernica has produced one of the war’s iconic images of suffering, five-year-old Omran Daqneesh, sitting in an ambulance, dazed and bloodied, white from masonry dust. This one unbearably distressing picture reignited the debate over whether the West can stand aside while Syria bleeds.

Paul Wood and Andrew Mitchell MP on the situation in Syria

But the inescapable fact for those who want to intervene — or intervene further — is that Aleppo’s anguish is not the work of the Syrian air force alone; Russian planes are attacking too. In fact, Russian pilots instil even more terror than the regime’s. They fly so high that people on the ground hear the jet’s engines only after the bomb has exploded — there is no warning of the attack. And the Russians drop huge bombs — ‘bunker-busters’ — that can level a whole building. ‘It destroys everything,’ Ismail said.

Halting the slaughter would mean grounding Russian planes as well as Syrian. The chairman of the US Joint Chiefs, General Joseph Dunford, was asked about a no-fly zone when he testified to senators last month. He told them, bluntly: ‘For us to control all the airspace in Syria would require us to go to war with Syria and Russia.’

The question now is: could Syria start world war three? Relations between America and Russia are worse than at any time since the Cold War. The US broke off its Syria talks with Russia this week. Russia, meanwhile, is ramping up its angry rhetoric, accusing America of directing a ‘terrorist internationale’ to prevent the Assad-Putin alliance from winning. The US ambassador to the UN, Samantha Power, accused Russia of ‘barbarism’.

The British are involved too, providing RAF support for America’s air campaign and firing more rhetorical missiles towards Moscow. On Tuesday, Theresa May accused Russia of supporting the regime’s ‘surrender or starve policy’ and called on Russia to allow in humanitarian aid. Russian war planes had been blamed for the bombing of an aid convoy into Aleppo as the ceasefire collapsed.

The Obama administration rejected a plan for a no-fly zone back in 2013. The Joint Chiefs reportedly told the White House that it would take 70,000 US military personnel to implement a no-fly zone over Syria. It was a number so large you might almost think it was deliberately intended to alarm the politicians. President Obama, eager to see out his last days in office without another major foreign policy crisis, did not need much persuading.

‘The logistics are enormous,’ said David Deptula, a retired US air force general who ran the no-fly zone over Iraq. Syria had much better air defences than Saddam’s Iraq, he said, and his list of what would be required was long: an array of bombers, fighters and refuelling aircraft, as well as cruise missiles. But he told me: ‘If you want to stop the Syrians from dropping barrel bombs off of heli-copters on to innocent men, women and children, there are other ways to prevent it which are a heck of a lot more efficient, rapid and less costly… direct attacks on the Syrian helicopter and fixed-wing forces. That can be done in 24 hours… That’s not a no-fly zone — that’s a focused attack… which is an act of war.’

The general was not himself arguing to bomb Syria’s airfields. ‘Why is it the-United States’s responsibility and not the United Nations’? What is the critical US-national security interest that risks the spilling of the blood of America’s sons and daughters? What threat has Assad presented to the United States?’

President Assad has survived in large part because of Russia. The assistance started early. Russia’s foreign minister, Sergey Lavrov, visited Syria in 2012, ostensibly to seek a peaceful resolution of the crisis. But he brought with him a large military and intelligence team, a western diplomat told me. They unrolled detailed maps of the country and, with their experience in Chechnya, instructed the Syrians in counter-insurgency. Then, last year, the Russians started bombing, after a period in which it looked as if Assad might fall. No one believes that will happen now.

One story illustrates how vital the relationship is to the regime. Over the summer Russia’s defence minister, Sergey Shoigu, visited the Russian airbase at Khmeimim in Latakia. Assad was there to meet him. 'Oh, it’s you,' Syria’s president is supposed to have said when the minister, in tan battle dress, strode into the room. The Russians had not even told him who was coming. They had summoned him, and that was enough.

The Russians know better than anyone how weak the regime’s military really is. Earlier this year, a senior Russian official astonished a visiting American delegation by telling them that the Syrian army could field only 6,000 ‘capable and loyal’ troops for a big operation. Officially, the Syrian Arab Army is 125,000 strong.

‘The paper strength of the Syrian army is meaningless,’ said Tobias Schneider, a German military analyst who has done some of the best work on the regime’s forces. ‘The only thing … is how much money they pay a month to somebody. Offensives in this war are 1,000 people. Anybody who actually had the numbers they claim would be able to capture the whole of Syria.’ Schneider has been getting leaked documents from inside the Syrian military in Hama. What is happening there is revealing. Hama is the base of one of the regime’s most effective militias, the so-called Tiger Forces. A leading member of the Tigers was a ‘powerful thug’ called Ali-Shelli, Schneider told me. Shelli apparently went too far, even for the regime, and was thrown into jail for ‘looting and plundering citizens at a checkpoint’, as the official report put it.

But Shelli was quickly released. And nothing happened to him when months later his men were caught smuggling weapons to sell to Isis, hidden under sacks of wheat in the back of a truck. The regime needed him. Three more of the regime’s militia commanders in Hama were supposed to be in jail, Schneider said. When the rebels launched an offensive in Hama last month, the regime’s defenders were warlords, smugglers, criminals, kidnappers and extortionists. ‘If you strip away all the trappings of a state, the way the war has done in Syria, this is what’s left. They control the state; they are the regime.’

The regular forces are no less corrupt. When the rebels started an offensive against the Alawite heartland of Latakia two month’s ago, they bribed the army officers in charge of a village called Kansaba. The officers abandoned their positions, leaving their soldiers behind, at least according to the furious charges of regime supporters on social media. ‘The officers left the fighting areas without any ammunition,’ a pro-regime Facebook page said, accusing these ‘users and defeatists’ of heading to ‘luxurious villas’ to consort with prostitutes.

The regime tried and failed to retake Kansaba with another semi-criminal militia, the Desert Hawks, until they finally succeeded using Afghan Shiite mercenaries. The coming regime offensive against Aleppo will be no different. Many of the troops massing outside the city — and according to Ismail, already entering it — are foreigners: fighters from the Iranian Republican guards and the Shiite militia they support in Lebanon, Hezbollah; Iraqi Shiite militias, and Afghan mercenaries. Iranian help on the ground has been just as vital to the regime as Russian help in the air.

What all this tells us is that the Syrian army is not necessarily the strong institution that could ensure order if the US does succeed in toppling President Assad. The regime’s forces are in many ways a mirror image of the rebels: a gang of militias more than an army, riddled with criminals, preying on their own people as much as defending them. Neither side is strong enough for victory, so the fighting continues, with 400,000 dead so far.

What would the next American president do? Donald Trump’s plan is secret but, given his admiration for President Putin, he may well side with Russia and back the regime, while continuing to bomb Isis. Hillary Clinton would continue to arm the rebels, though perhaps opening a tap that has only been dripping slowly under President Obama. She has also said she is in favour of a no-fly zone, though she has given no details.

One presidential candidate you probably haven’t heard of, Evan McMullin, has been discussing how this would work. McMullin is an independent Republican from the Stop Trump movement. He is polling in the low single digits and not on the ballot in many states, but his backers hope he will get just enough votes to deprive Trump of victory.

McMullin, a former CIA officer in Iraq, is a believer in the ‘efficient, rapid and cheap’ plan explained by General Deptula to bomb President Assad’s airfields. Crucially, he says, Russia should first be given a warning, so they can get out of the way: ‘Russia just simply doesn’t have the strength that it used to. Its military isn’t as strong as it was during the Afghan war… They’re not able to project force in contest; they’re able to project force when they’re unopposed… We’re calling their bluff.’

The Russian military has now announced that it is sending a battery of the S300 air defence missiles to Syria. This is not world war three, but it is starting to look like a new Cold War. Hillary Clinton’s no-fly zone rests on the belief that Vladimir Putin will deflate like a punctured balloon when challenged. But what if he does not?

Relations between America and Russia are now worse than at any time since the Cold War

Paul Wood spent four years covering Syria’s civil war for the BBC and is a fellow at the New America foundation in Washington.