Uguccione della Faggiuola, the Imperial Vicar of Genoa, remained, as Imperial legate, Podesta, Captain of the People, and Elector, bringing with him one thousand German horse.  The rest of the army of Henry returned over the Alps.  Pisa thought herself on the verge of ruin; she must make terms with her foes.  This being done, there appeared to be no further need for Uguccione, whose German troops were expensive, and whose presence did but anger the Guelphs.  Uguccione was a man of enormous strength, brave, too, and resolute, swift to decide an issue, wise in council, but a barbarian.  What had he to do with peace.  His business was war, as he very soon let the Pisans know.  Nor were they slow to take him at his word.  Pisa was never beaten.  Uguccione marched through the streets with the living eagles of the Empire borne before him.  Before long he had deprived the Guelphs of power, and was practically tyrant of Pisa.  Everything now seemed to depend on victory.  Lucca scarcely ten miles away, Guelph by tradition and hatred of Pisa, was in an uproar.  Uguccione saw his chance and took it; he flung himself on the city and delivered it up to its own factions while the Pisans sacked it.  Nor did they spare the place.  The spoil was enormous; among the rest, a large sum belonging to the Pope fell into their hands.  Florence and her allies sprang to arms.  Uguccione took up the challenge, burnt the lands of Pistoja and San Miniato al Tedesco, ravaged the vineyards of Volterra, seized the fortresses of Val di Nievole, and at last besieged Montecatini.

It was now that the Ghibellines of Lucca with Castruccio Castracani joined Uguccione.  They met the army of Florence at Montecatini.  Machiavelli states that Uguccione fell ill, and had no part in the battle, which was won by Castruccio.  Villari, however, gives the glory to Uguccione.

It might seem that Uguccione, whether ill or not on the day of battle, was jealous, and perhaps afraid, of Castruccio.  Certainly he plotted against him, sending his son Nerli to Lucca with orders to trap Castruccio and imprison him; which was done.  Nerli, however, wanted resolution to kill him; and his father hearing this, set out from Pisa with four hundred horse to take the matter in hand.  The Pisans, who were by this time completely enslaved by Uguccione, seized the opportunity to rise.  Macchiavelli tells us “they cut his Deputies’ throats, and slew all his Family.  Now, that he might be sure they were in earnest, they chose the Conte de Gherardesca, and made him their Governor.”  When Uguccione got to Lucca he found the city in an uproar, and the people demanding the release of Castruccio.  This he was compelled to allow.  With Castruccio at liberty, Lucca was too hot for him, and he fled into Lombardy to the Lords of Scala, where no long time after, he died.

After the great victory of Montecatini, Gherardesca and Castruccio soon came to terms with the Guelphs; and all that Pisa really seems to have gained by the war was that she was compelled to build a hospital and chapel for the repose of the souls of the dead at Montecatini.  This chapel, hidden away in the Casa dei Trovatelli at the top of Via S. Maria in Pisa, became a glorious monument of the victory of Pisa over Florence.