AFTER months of diplomatic wrangling America’s secretary of state, John Kerry, hoped he had finally struck a deal with Russia that would help end the war in Syria, which has killed perhaps half a million people. For the plan to work, both sides needed to lay down their weapons for one week and allow aid into besieged parts of the country. If that happened the truce would then be extended, paving the way for Russia and America to launch joint military action against Islamic State and Jabhat Fatah al-Sham (JFS), a terrorist group and former al-Qaeda offshoot.

But the plan never got that far. Although the fighting ebbed, the Syrian government blocked most aid deliveries into rebel-held areas, and stripped vital medical supplies from the few that it did allow across the front lines. On September 19th the Syrian regime refused to extend the seven-day ceasefire, accusing rebels of failing to uphold their side and citing an air strike by American and coalition forces that mistakenly killed 62 Syrian soldiers.

But the real breach came soon after Russian and Syrian warplanes went back into action, pounding rebel-held neighbourhoods in the northern city of Aleppo. A UN aid convoy was bombed—the first attack of its kind since the start of the war. American officials said Russian jets were to blame, citing radar tracks that showed them above the convoy when it was hit. Russia denied it, claiming variously that the trucks had simply caught fire or been shelled.

Ban Ki-moon, the secretary-general of the UN, called it a “sickening, savage and apparently deliberate attack”. His officials said that if the convoy was deliberately targeted, that would amount to a war crime. (Syria has seen many war crimes in the past five years.) A day later aircraft bombed a mobile clinic in a rebel-held part of Aleppo, killing four medical staff. The UN and several other humanitarian groups said they were suspending aid convoys.

A ceasefire that had taken months to negotiate took only hours to unravel. “They were unloading the aid in a warehouse when the bombs hit. I spent the night pulling the dead out,” says Ammar al-Selmo, the director of Aleppo’s White Helmets, a volunteer civil defence force that works in rebel-held areas, of the attack on the UN aid convoy.

Mr Kerry, whose plan probably represented America’s last real diplomatic effort under the presidency of Barack Obama to slow the killing, is still scrambling to salvage what is left of it. But unless he can convince his Russian counterpart, Sergei Lavrov, to renew the ceasefire (and persuade the Syrian regime to ground its aircraft, an appeal Mr Kerry made at the UN on September 21st), then dark days lie ahead.

Fighting will probably intensify as the Syrian government, backed by Iran and Russia, doubles down on its efforts to crush rebels in eastern Aleppo, their only major urban stronghold. The fall of Aleppo would at last give President Bashar al-Assad what he craves: dominion over the country’s main cities, industrial hubs and transport links, including access to the sea (see map). In a troubling sign of the fighting to come, Iran has apparently taken advantage of the truce to reinforce its militias around the city. America is now considering arming Kurds in northern Syria, which would pit it against Turkey, a NATO ally.

Rebel forces are also preparing for another round of fighting. A long-discussed merger between more mainstream Islamist groups and JFS is back on the cards.

Rebels in arms

“The merger is a goal for all the Syrian rebel factions. If it was successfully done it would mean a significant turn in the path of the revolution,” says Captain Abdul Salam Abdul Razaq, a military spokesman for Nour al-Din al-Zinki, a key rebel group in northern Syria that once received American military support.

The merger talks are still at an early stage. Mainstream rebels fear that forming a coalition with JFS would expose them to American air strikes. The two sides also disagree on their visions for Syria’s future. Still, if a fresh round of fighting begins, then a stronger military alliance of Islamist factions would stand a better chance of fending off Mr Assad’s advances.

Such a deal would probably torpedo Mr Kerry’s ceasefire plan as well as America’s broader aim of trying to arm moderate rebels to fight against Islamic State. Moscow already accuses America of failing to separate mainstream rebel factions from “terrorist” groups like JFS, a precondition of any joint military action between the two countries. Rebels have so far been reluctant to separate, fearing that doing so will only result in them ceding territory to the Syrian army.

Even if Mr Kerry persuades the warring parties to extend the ceasefire, there is little chance that peace talks will yield results. The political opposition to Mr Assad is weak and the rebels’ trust in the UN has reached a new nadir. America has little leverage over Russia, Iran or Syria.

“The longer this goes on for, the more difficult it will be to hold the centre ground together,” says Salman Shaikh, a former UN official and expert on the Middle East. “One consequence is likely to be the further radicalisation of the mainstream opposition ... a five-year conflict could easily become a 10-year conflict.”