It is difficult to draw a clear distinction between defence against external threats and that against internal insurgency. Such distinctions have rarely been drawn in history. City walls were intended as much to keep citizens in, and accounted for, as to keep enemies out. The wall (as has already been noted in Chapter 2) had a practical, as well as symbolic, jurisdictional purpose, enabling the urban authorities to exercise a control over the movement of goods and people, and thus served police, customs, fiscal and immigration purposes (as such flows could be channelled through the limited number of gates which would be opened and guarded at specific times). Thus the distinction between police and military structures was generally blurred, with the same forces being called upon to perform both functions. Indeed, the only distinction between the modern situation—especially in countries with the Anglo-American aversion to paramilitary police forces (Gellner 1974)—and that which prevailed in most countries until the last century, is that the military now operate ‘in support of the civil power’, whereas previously they were frequently the only effective instrument of that power.
External aggression and internal lawlessness have frequently been related—either as causes and effect, or as combined strategies in an assault upon the city. For much of the Middle Ages, the technical superiority of urban defence over urban attack determined that a strongly walled city was more likely to succumb to internal subversion, or a breakdown of internal order and resolve among the defenders, than to a direct assault on the fortifications from outside. General Franco’s boast of a fifth column inside Madrid to supplement his four advancing upon it in 1936, was an echo of the hopes of most besieging forces during the preceding thousand years.
Although internal and external defence have frequently proved to be indivisible, it is nevertheless useful to consider separately the internal security of the city and attempts to subvert it through insurgency. The propositions underlying this chapter are that cities have proved especially important to insurgency, providing a particularly ‘favourable morphological and social environment for the generation and success of such activities, and