When viewed in terms of the technical development of projectile weapons, there is a clear logical line of progression that begins with the muscle-propelled stone or spear and ends with the rocket-delivered nuclear warhead. The bomber and the ballistic missile are thus only an extension of artillery, to which cities have for some 500 years been subjected. Such a technical continuum, however, conceals a distinctly new strategic role of cities, and a new set of reactions to that newly imposed role—introduced by the possibility of their attack from the air.
In 1849 the Austrian army, when somewhat unsuccessfully engaged in besieging Venice, tested the idea (first considered in the 1780s) of dropping explosive projectiles from balloons drifting across the city. This was correctly hailed at the time as the beginning of a new era in urban warfare, despite the failure to inflict either casualties or damage, or even hit the city with all but one bomb. Three conclusions were immediately drawn by attackers, defenders and neutral observers alike. First, a new front had been opened—the skies above the cities—in which the attackers had largely unrestricted and unchallenged freedom of movement and selection of targets, while the defenders were effectively defenceless. (Balloons had long been used for reconnaissance—as in the 1796 siege of Mainz and artillery spotting, but not in such a directly offensive role.) Second, the objective was clearly recognized as being the intimidation of the civilian population, removing at a stroke the centuries-long distinction between legitimate military, and immune non-military, targets, placing cities in the front line of future engagements and placing their civilian populations in as much danger as military personnel. Of course, bombardment intended to intimidate a civilian population was not new: Britain’s incendiary rocket attack on Copenhagen in 1807 (Lindeberg 1974) is a textbook example of its deliberate and successful use, but it remained an option and was not an inevitable use of the weapon. Third, Radetsky’s use of aerial bombardment was greeted with a near universal moral outcry, and there was much soul-searching among