Pasquale Villari,
The Two First Centuries of Florentine History
“ For some time past it had been the aim of Florence to obtain a monopoly of these affairs, and this was one reason why she had always adhered to the Guelphs. She had had frequent disputes with Arezzo, Volterra, and above all with Sienna, as being the most powerful city on the road to Rome. So the two rivals were perpetually stirred to fresh and fiercer strife. So, too, before long, the irresistible need of Florence for free communication with the sea became the chief cause of her equally long and sanguinary wars with Pisa, the city barring her way to the coast. ”
