The most difficult part of a peace process is getting the main combatants to acknowledge that a stalemate exists on the ground and that continued fighting has no immediate purpose. Rarely does one party enter negotiations when it believes it can achieve some advantage through continued fighting. Often, just before talks begin, conflict intensifies as combatants try to gain some advantage going into negotiations.

That appears to be occurring now as the Syrian government steps up its campaign to liberate the city of Aleppo from rebel forces before the next round of United Nations-sponsored peace talks begin. In recent weeks, fighting has intensified, with, once again, reports of the use of chemical weapons against civilian populations. The atrocities must stop, the fighting must end and the peace process must be given every opportunity to proceed.

Aleppo in 2011 was Syria’s second-largest city, the country’s commercial center and a UNESCO World Heritage site. It became a target of Syrian rebels in July 2012 and portions of the city fell under their control within weeks. A government siege continued, and the periodic offensives have been launched, but the rebels have maintained control of the eastern part of the city.

During that time, the population has fallen; it is estimated that between 200,000 and 300,000 people remain. Living conditions are difficult, as would be expected after four years of bombardment and isolation. The city lacks food, water, fuel and medical supplies. According to some reports, government forces have bombarded parts of the city every day for nearly three months. During that time, nongovernmental groups estimate that at least 6,000 people have either been killed or injured. For those who survive, the odds of staying alive are getting longer. Ambulances have no gasoline and hospitals have no power. According to the World Food Program, the price of rice has increased by 32 percent in three months and is 142 percent higher than a year ago.

Government forces have opened corridors for those stuck in the city to leave, but most observers believe that the corridors are really for resupply to prepare for a final assault. Even if the offer to evacuate citizens is sincere, there is great concern about what will follow: In other cases when the government has offered to evacuate people from rebel-held areas, men and women have been separated and the men disappear.

During the most recent offensive, a Russian helicopter was downed with the loss of five lives. Shortly after, there were reports of attacks with chemical weapons. Opposition forces accused the government of responsibility, a charge that Damascus denied. It then accused the rebels of firing rockets with gas on government-held areas of the city. That accusation was dismissed as quickly by the rebels.

There is little doubt that chemicals were used. Videos show individuals having difficulty breathing, and there are eyewitness reports from a doctor who said the symptoms were consistent with “someone who has suffered from chlorine poisoning.” Other witnesses spoke of symptoms consistent with a gas attack. One human rights group counted nine suspected chlorine gas attacks in that area since the conflict began.

U.N. investigators established that sarin was used in Syria in 2013. The Syrian government was accused of that attack — a charge it denied — which killed nearly 1,500 people, 426 of them children. While that use of chemical weapons did not trigger U.S. intervention as had been threatened by Washington, it did result in a commitment by the Syrian government to destroy its declared stockpile of chemical weapons, a process that was declared completed in early 2016.

The Organization for the Prohibition of Chemical Weapons confirmed that mustard gas was used in March 2015 and again in August of that year in Syria, and against Kurdish forces in Iraq in August 2015, most likely by Islamic militants. Many experts believe that use of those weapons indicates that the destruction of the Syrian stockpile was not complete; some counter that Islamic rebels made the weapons from components available in captured territory. Either way, a dangerous threshold has been crossed and the taboo against the use of chemical weapons has been eroded. This must be corrected.

The use of chemical weapons also complicates the peace process. If the Syrian government is responsible, then it will dig in its heels even more resolutely to ensure that it is not held accountable for crimes against humanity. In other words, a settlement that removes Syrian President Bashar Assad has become even harder to achieve.

Given the atrocities already committed by rebel forces, one more crime against humanity is likely to have little impact on their thinking. But if the group did in fact use chemical weapons, then it is even harder to accept them as a legitimate negotiating partner and the Syrian government and its Russian allies will be even more intransigent in the negotiations. As the talks drag on, the people of Aleppo will continue to suffer.

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