LONDON—To the modern imagination, there is nothing so terrifying as a high-tech weapon. Our horror movies are filled with crashing planes, engineered viruses, and rogue computers that have taken charge of spaceships. Our security nightmares have long been focused on nuclear, chemical, and biological weapons. The phrase we use to describe these contemporary horrors, “weapons of mass destruction,” implies something diabolically modern, something so innovative that it can kill thousands in an instant.

But in our obsession with what is terrifyingly new, it seems we have forgotten that there are much older ways to kill large numbers of people. Certainly there is one weapon of mass destruction so ancient and low-tech that it doesn’t even involve gunpowder, let alone the lethal tools we so dismissively refer to as conventional weapons. This method is called “starvation.”

In medieval Europe, starvation was the de facto consequence of a siege. An army would surround a castle or a walled town, prevent food from entering—and then wait. The inhabitants would grow weak. They would lose their hair and their teeth. They would then surrender, or die in large numbers.

In the 20th century, dictators used starvation not just as a battle tactic but also to murder people who did not fit into their vision of an ideal society. Before resorting to more “industrial” methods, Hitler used starvation to kill Jews: Nazi soldiers shut them in ghettos, closed the doors, and shot children who tried to smuggle food in through the sewers. Stalin used starvation to kill Ukrainian peasants: Soviet soldiers confiscated their grain, forcibly removed food from their larders, and blocked roads so nothing could reach them. As in the Middle Ages, the Jews of the Łódź ghetto and the peasants of Kharkiv district grew weak, lost their hair and teeth, and then died. Millions of people were thus murdered, without a whiff of sarin gas or a particle of plutonium.

Nowadays, “death by forced starvation” sounds like something from an old newsreel. But it is not. Right now, in the 21st century, the Syrian dictator Bashar al-Assad is once again making use of it. While the international community is haggling over his chemical weapons, the stuff of modern nightmares, he is following the example of his medieval and his 20th-century predecessors and deliberately starving thousands of people to death.

Because he says he doesn’t want to feed armed rebels, trucks filled with food aid now sit outside the besieged city of Homs, where an unknown number of civilians have had no supplies for many weeks. The effects are the same as they were in the Łódź ghetto. A Dutch priest who has remained inside the city has described people literally going mad from hunger: “Infants are suffering the most. Nursing mothers can’t feed their babies as they are too weak from hunger. We search everywhere for milk, and when we find it we mix it with water.”

At the same time, about 20,000 refugees living in the Yarmouk camp near Damascus are also growing weak, losing hair and teeth, and dying. During the Ukrainian famine of the early 1930s, starving people ate insects, leaves, and grass to stay alive. In Yarmouk people are now eating cats, cactus, and grass to stay alive. One witness has said that some people there are consuming only water. “Sometimes we do this … drink some water with some sugar or some salt and go back to sleep. But when you go to the street you will find maybe the people next door … they’re dead.” Here too the government refuses to let in relief vehicles, on the grounds that “terrorists” have sought refuge inside.

Worse is to come, for as the war continues, the politics of starvation grow more complex. In revenge for the siege of Homs, Syrian rebels have besieged a handful of villages near Aleppo. In other places government blockades are looser, though everyone knows they could be tightened if need be. In this war starvation is a particularly useful battle tactic and a political tool. Not only can it help armies growing short of weapons, starvation can literally eliminate opponents. People who are starving—or dead—will not fight back.

Above all, starvation, unlike chemical weapons, is useful in this war because it does not rise to the highest levels of international concern. A handful of foreign ministers have condemned the use of starvation in Syria, but so far it has not provoked public campaigns or mass media coverage. It has not given rise to a debate about humanitarian intervention. And this is why it continues: For the perpetrators, it’s a slower, safer form of murder than a nuclear bomb. Which doesn’t mean that it is any less lethal for the victims.