Yet, on the other hand, if we make abstraction of pope and emperor, and consider only the nobles and the citizens, it is clear that they had among them the seeds of strife immeasurable. The nobles were by training and habit centres of violence.[606] Their mutual feuds, always tending to involve the citizens, were a perpetual peril to order; and their disregard of law kept them as ready to make war on citizens or cities as on each other. Again and again they were violently expelled from every Lombard city, on the score of their gross and perpetual disorders; but they being the chief experts in military matters, they were always welcomed back again, because the burghers had need of them as leaders in the feuds of city with city, and of Guelphs with Ghibellines. So that yet again, if we put the nobles out of sight, the spirit of strife as between city and city was sufficient, as in ancient Greece, to make them all the prey of any invader with a free hand. They could not master the science of their problem, could not rise above the plane of primary tribal or local passion and jealousy; though within each city were faction hatreds as bitter as those between the cities as wholes. Already in the twelfth century we find Milan destroying Lodi and unwalling Como. Later, in the thirteenth century, Genoa ruins the naval power of Pisa,[607] then under the tyranny of Ugolino, in a war of commercial hatred, such as Pisa had before waged with Amalfi and with Lucca; in the fourteenth, Genoa and Venice again and again fight till both are exhausted, and Genoa accepts a lord to aid her in the struggle, Pisa doing likewise, and so recovering strength on land;[608] in the fifteenth and sixteenth, Florence spares no cost or effort to keep Pisa in subjection. This fatal policy, in turn, was the result of the frequent attempts of the Pisans to destroy Florentine trade by closing their port to it.[609] All along, inter-civic hates are in full flow through all the wars of Guelphs and Ghibellines; and the menace of neither French nor Spanish tyranny can finally unify the mutually repellent communities.