All the cities alike were at once industrial and military, with the exception of Rome; and for all alike a career of mere plunder was out of the question, though every city sought to enlarge its territory. Forcible unification could conceivably be wrought only by the emperor or the Papacy; and in the nature of things these powers became enemies, carrying feud into the heart of every city in Italy, as well as setting each on one or the other side according as the majority swayed for the moment. At times, as after the destruction of Milan by Frederick Barbarossa, hatred of the foreigner and despot could unite a number of cities in a powerful league; but though the emperor was worsted there was no excising the trouble of the separate interests of the bishops and the nobles, or that of the old jealousies and hatreds of many of the cities for each other. Pope Innocent IV, after the death of Frederick II in 1250, turned against the Papacy many of the Milanese by his arrogance. They had made immense sacrifices for the Guelph cause; and their reward was to be threatened with excommunication for an ecclesiastical dispute.[543] The Christian religion not only did not avail to make Italians less madly quarrelsome than pagan Greeks: it embittered and complicated every difference; and if the cities could have agreed to keep out the Germans, the Papacy would not have let them. Commonly it played them one against the other, preaching union only when there was a question of a crusade. Some writers, even non-Catholics, have spoken of the Papacy as a unifying factor in Italian life. Machiavelli, who was pretty well placed for knowing where the shoe pinched, repeatedly (_Istorie fiorentine_, l. i; _Discorsi sopra Tito Livio_, i, 12) speaks of it bitterly as being at all times the source of invasion and of disunion in Italy. This is substantially the view of Gregorovius (_Geschichte der Stadt Rom_, B. iv, cap. iii, § 3) as to the process in the city of Rome to begin with. So also Symonds: "The whole history of Italy proves that Machiavelli was right when he asserted that the Church had persistently maintained the nation in disunion for the furtherance of her own selfish ends" (_Renaissance in Italy: The Age of the Despots_, ed. 1907, p. 75).