GAZIANTEP, Turkey — As the self-proclaimed Islamic State, or ISIS, commits horrendous videotaped executions, it might seem to pose the greatest threat to Syrian civilians. In fact, that ignoble distinction belongs to the barrel bombs being dropped by the military of Syria’s president, Bashar al-Assad. The Islamic State has distracted us from this deadly reality.

Barrel bombs are improvised weapons: oil drums or similar canisters filled with explosives and metal fragments. They are dropped without guidance from helicopters hovering just above antiaircraft range, typically hitting the ground with huge explosions and the widespread diffusion of deadly shrapnel. They pulverize neighborhoods, destroy entire buildings and leave broad strips of death and destruction.

The Syrian military has dropped barrel bombs, sometimes dozens in one day, on opposition-held neighborhoods in Aleppo, Idlib, Dara’a and other cities and towns. They have pulverized markets, schools, hospitals and countless residences. Syrians have described to me the sheer terror of waiting the 30 seconds or so for the barrel bomb to tumble to earth from a helicopter hovering overhead, not knowing until near the very end where its deadly point of impact will be.

From the start of the war, the Assad government has pursued a murderous policy toward Syrian citizens who happen to live in areas that have been seized by opposition armed groups. The apparent aim is to kill and terrorize civilians (and destroy civilian structures) so as to drive civilians from opposition-held areas and to send a warning of the misery that attends anyone whose neighborhood is taken by opposition groups. Mr. Assad is thus pursuing the “total war” strategy that the Geneva Conventions and the laws of war flatly prohibit and criminalize.