In the beginning, it was not like this. Spreading terror was historically an instrument of state and military policy. When there were only foot soldiers to do it, they might be sent out to kill and rape, more or less at random, in a densely populated town or city either as revenge for some slight or as a lesson designed to cow those who survived into terrified submission. Soldiers terrified people and they created an atmosphere of terror.
The invention of air planes added new possibilities. In the 1920s, Arthur Harris - later "Bomber" Harris of Britain's World War Two Bomber Command - wrote this in a 1924 report from his RAF base in Iraq:
"They [ the Arabs and the Kurds] now know what real bombing means, in casualties and damage: they now know that within 45 minutes a full sized village, vide attached photos of Kushan-Al-Ajaza, can be practically wiped out and a third of its inhabitants killed or injured by four or five machines which offer them no real target, no opportunity for glory as warriors, no effective means of escape, and little chance of retaliation or loot such as an infantry column would afford them in producing a similar result" [ quoted from Y.Tanaka and M.Young, editors, Bombing Civilians (2009) page 21]
In other words, the British used aerial bombardment in 1920s Iraq - a new country which they created and controlled, one of the spoils of the 1914 - 18 war - to terrify and cow the local population.
But though we think a man a terrorist - occasionally a woman - who nowadays walks into a crowded Iraqi market and blows himself up along with as many other people as he can manage, we probably don't think that of the pilot of a small bi-plane dropping bombs from a great height - possibly even heaving them over the side of the plane by hand.
Partly it's to do with the fact that we are unnerved by the ideological fanaticism of a volunteer who deliberately blows himself up. In contrast, the RAF pilot did not choose his mission. He had signed up to serve and his country, as it happens, sent him to Iraq and told him to drop bombs on defenceless villagers. He was carrying out a policy of terror without being the maker of that policy.
In this case, the maker of the policy was Arthur Harris and his policy was not uncontroversial. There were RAF officers and local civil servants who were opposed to what he was developing as an instrument of Imperial policy. But in the end, the extremist Harris won out over the moderates who opposed his policy of indiscriminate violence.
Fifty years later, bombing civilians from a great height had become central to the Imperial military strategy of the United States in Vietnam, Cambodia and Laos. The planes were bigger, the bombs bigger and more lethal (and some of them chemical: remember Napalm?), the accuracy of bombing probably as dismal as ever: American pilots off their heads on the illegal Substances of the era.
The strategy was to break the will of the peasants who happened to survive the fury of the American onslaught, to cow them into submission.
Interestingly, it didn't seem to work.