2. Is counter population warfare acceptable?

“Any act of war aimed indiscriminately at the destruction of entire cities or extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation.” Vatican II, Church in the Modern World (1965) 80.
“Not even by the broadest definition can one rationally consider combatants entire classes of human beings such as school children, hospital patients, the elderly, the ill, the average industrial worker producing goods not directly related to military purposes, farmers and many others. They may never be directly attacked.” U.S. Bishops, The Challenge of Peace (1983) 108.
“Under no circumstances may nuclear weapons or other instruments of mass slaughter be used for the purpose of destroying population centers or other predominantly civilian targets.” U.S. Bishops, The Challenge of Peace (1983) 147.

Reflections on counter population warfare.
a) Why is the destruction of entire cities to be condemned?
b) If we are not allowed to destroy cities, how are we to defend ourselves against an enemy that has decided to attempt to destroy our cities?