GAZIANTEP, Turkey - The three young Syrian women had managed to escape from the rebel-held section of Aleppo for a few days' rest across the border in Turkey. Asma, 26, a university graduate in English literature, has been volunteering for the past two years as a nurse in a field hospital, treating civilian victims of the war, which has divided Aleppo into conflict zones held by the rebels and the regime.

Salam, 30, and Islam, 28, sisters who were teachers before the war, are volunteers in an orphanage that shelters 650 children who lost parents in the fighting. "These children are only a fraction of the number of war orphans," Salam told me. It was hard to believe that these fresh-faced, smiling women, their faces framed by head scarves, had lived for years under bombardments.

But just after they arrived in Gaziantep, the Assad regime began dropping "barrel bombs" filled with hundreds of pounds of explosives and shrapnel on apartment buildings in their neighborhood, killing hundreds of civilians and wounding hundreds more. The three were planning to rush back into the mayhem.

"There are not enough doctors or nurses or supplies," Asma explained as we sat over a simple lunch of bread, cheese, and cucumbers in the apartment of friends who had escaped from Aleppo. "They are operating in the corridors, cutting off limbs without anesthetic." A school had been hit, and many of the new victims were children.

No surprise. Bashar al-Assad's war against his own people is a deliberate war against civilians and children. The greatest humanitarian catastrophe of the 21st century thus far is unfolding in Syria. The war crimes committed by Assad rival anything seen in Darfur or Bosnia.

Yet the international community has failed to take steps that could slow or halt these war crimes. And the Obama administration has unwittingly helped Assad continue slaughtering civilians.

This has to stop.

According to the Oxford Research Group, at least 11,000 of the more than 113,000 known dead were younger than 18; of those child victims, more than 70 percent were killed by bombs or artillery shells. There were also 764 cases of summary execution of children and 389 of sniper fire with clear evidence that children were targeted. (Indeed, the entire Syrian revolt was sparked in late 2011 by the arrest and torture of a group of children for writing anti-Assad graffiti on a wall.)

Indiscriminate bombing and shelling have been Assad's weapons of choice for depopulating whole urban neighborhoods and towns under rebel control. The result is that nearly one-third of Syria's 23 million people are either refugees in neighboring countries or internally displaced, living in schools, in mosques, or on the ground.

As winter sets in, Assad is preventing food and medicine from reaching 250,000 starving civilians in besieged areas surrounded by regime forces. As I heard from doctors in Gaziantep, the dictator is abetting a growing polio epidemic, which is on the verge of exploding in the rebel-held north; regime policy ensures that the most endangered children lack access to the vaccine.

U.N. investigators have alleged that the Syrian government has carried out "a campaign of terror" against civilians that includes widespread abductions and disappearances. The top U.N. human rights official, Navi Pillay, says evidence links Assad to crimes against humanity, including horrendous torture and rapes in regime prisons. (Yes, some opposition groups have also committed war crimes, but they amount to a tiny fraction of the regime's atrocities.)

Yet Assad is sitting pretty in the run-up to peace talks with the opposition, to be held in Switzerland on Jan. 22. His Russian backers blocked a U.N. Security Council statement last week that would have condemned the barrel-bomb attacks. Their veto makes it impossible to refer Assad to the International Criminal Court for crimes against humanity.

Obama's decision to forgo a strike on the regime's military sites in favor of a deal to destroy its chemical weapons gave the dictator a green light to slaughter civilians by other methods. As Assad ups the killing before peace talks, Obama's only card is to ask Moscow to rein him in.

If Russia (and Iran) will not force Assad to cease his slaughter of civilians and declare a humanitarian truce before the beginning of peace talks, the talks are pointless. Unless the administration stands by this precondition, it will be complicit in Assad's crimes.

Americans may be indifferent to the slaughter of Syrian civilians because they view the war as too confusing to grasp. But if they had the chance to talk to Asma, Salam, Islam, and all the brave Syrian civilian activists I met in Gaziantep, they would understand the essence of this fight. These young people have exposed themselves to mortal danger to save civilian lives.

This war is about a dictator who will go to any lengths to retain power, and who has triggered a humanitarian catastrophe that destabilizes the whole region. Assad's reign of terror has fueled a rise of radical Islam that will threaten the entire Middle East - and the West.

Peace talks in Geneva are unlikely to end the war, but at least they might stop Assad's assault on civilians. If Obama cannot persuade the Russians to force Assad to permit the delivery of food to the starving and vaccine to the children, the U.S. delegation should stay home.