A prominent Syrian rebel commander with the nom de guerre Abu Sakkar recently appeared on YouTube cutting open the chest of a dead government soldier, pulling something out of it—the heart or perhaps a lung—and taking a bite. Abu Sakkar claimed that such cannibalism was an appropriate psychological payback for the crimes of Bashar Assad's troops, who have recorded videos of their own atrocities. "I swear to God we will eat your hearts and your livers," Abu Sakkar promises in the gruesome clip.
Barbarity is now commonplace in the Syrian war. Some 80,000 Syrians have been killed since the Arab Spring arrived in March 2011, and unknown numbers have been tortured and maimed. Many expected that Assad would follow the relatively rapid demise of fellow Arab kleptocrats like Hosni Mubarak in Egypt, Moammar Gadhafi in Libya and Tunisia's Zine El Abidine Ben Ali. But buoyed by Russian arms, Iranian money and agents, and Hezbollah terrorists, Assad has hung on as more than a million Syrians have fled the country.
According to reports, he is close to achieving a stalemate, and he is even capable of launching aggressive counteroffensives, as in the heavy recent fighting for Qusayr near the Lebanon border. Thus atrocities and counter-atrocities grow—and the world wonders why Syrian fighters seem especially prone to premodern brutality.
The truth is that atrocity is common in war, ancient and modern. King Xerxes had the slain Spartan hero Leonidas decapitated after the Greek defeat at Thermopylae and his head impaled on a stake. One of the most chilling passages in William H. Prescott's classic history of the ruthless Spanish destruction of the Aztec Empire is the sight of 62 captured conquistadors atop the Great Pyramid at Tenochtitlan in ceremonial garb, ready to be sacrificed by having their hearts torn out.
The habit of Vlad III, Prince of Walachia, of impaling his captured enemies inspired Bram Stoker's "Dracula." E.B. Sledge's brilliant World War II memoir of island fighting in the Pacific, "With the Old Breed," cites incidents of lopped off genitals. And while Sledge makes the case that the Japanese were more prone to mutilate the dead than were the Americans, he saw enough barbarity among his fellow Marines to leave him depressed over human depravity.
Some wars are more likely to see routine sadism of the Syrian type than are others. The humiliation of Iraqi prisoners at Abu Ghraib stirred revulsion among the American public, but on history's scale of atrocities it was not remotely in the same league as what occurred in the Rwanda genocide in 1994, the Iran-Iraq War or the Soviet fighting in Afghanistan during the 1980s, or the Balkan nightmare of the 1990s.
One way to ensure brutal cycles of violence is to prolong fighting. Mubarak was toppled quickly. Had he turned the army against the protesters and incited a civil war, the grotesque episodes we see in Syria might have become commonplace in Egypt. The Six-Day War of 1967, in which Israel fought back against Egypt, Jordan and Syria, lacked the horrendous violence of the drawn-out conflict in South Lebanon between Israel and Hezbollah, the PLO and others that lasted from 1982 to 2000.
Unconventional and undeclared fighting—marked by terrorism, insurgency and Mogadishu-like irregulars—are also force multipliers of atrocity. Professional soldiers, while adept at industrialized brutality, are still more likely than rebels or militias to accept a rough code of conduct. During trench warfare or armor-led attacks and counterattacks, civilians were not as likely targeted or to take up arms. World War I was far more lethal to American troops than was the much longer Philippine insurrection between 1899 and 1913. Yet the latter's primitive slaughter outraged the American people in a way that even the horrendous machine-gunning in Belgium and France had not.
Other criteria also influence the levels of atrocity. Consensual societies are more likely to hold their soldiers accountable, given a free electorate and press. The Allies firebombed civilian centers during World War II, but setting up anything like Hitler's industrial death camps would have been virtually impossible, even if the Allies had, unthinkably, been so disposed. Kaiser Wilhelm II was no liberal reformer, yet even the shadow of a Reichstag in 1914 Germany made imperial soldiers less likely to torture and maim than were their sons in the totalitarian-driven Third Reich.
Finally, "war" is a loose abstraction that can include everything from the Falklands campaign ("a fight between two bald men over a comb" in the famous quip of the Argentine novelist Jorge Luis Borges) to the horrific 14-year Japanese on-and-off war in Manchuria that eventually saw 10 million perish. When the struggle is not prompted by an uninhabited rocky island, a disputed border, or a soccer match, but rather involves medieval Christian Crusaders versus Muslims for the religious future of the Middle East, the fate of the American or Australian frontier, or the extinction of millions in Europe, these total wars can become totally barbarous given that the alternative to victory is not defeat, but often extinction or slavery.
The Syrian war meets many of military history's criteria of barbarism. We are witnessing a third year of the fighting, marked by roving bands rather than a formal duel between uniformed soldiers squared off on either side of no-man's land.
Neither side—if there are indeed two sides, rather than four or five—is democratic. Both Syrian soldiers and militias know there is scant chance of postwar punishment for their barbarism. The killing is not merely over the future of Syria: It is also a religious struggle between Shiite and Sunni Muslims, framed by a parallel fight between Baathist authoritarianism and theocratic Islamism.
The losers surely expect something worse than defeat—all they need to do is remember Hafez Assad's 1982 massacre of rebels in Hama and the city's near-razing to sense what might await. There will be more Abu Sakkars before this savage war is over.
Mr. Hanson is a senior fellow at Stanford University's Hoover Institution. His most recent book, "The Savior Generals: How Five Great Commanders Saved Wars That Were Lost—From Ancient Greece to Iraq," is just out from Bloomsbury.