Castruccio, having returned to Lucca in far higher estimation that he left it, did not omit to use all the means in his power to gain as many friends as he could, neglecting none of those arts which are necessary for that purpose. About this time Messer Francesco died, leaving a son thirteen years of age named Pagolo, and having appointed Castruccio to be his son's tutor and administrator of his estate. Before he died Francesco called Castruccio to him, and prayed him to show Pagolo that goodwill which he (Francesco) had always shown to HIM, and to render to the son the gratitude which he had not been able to repay to the father. Upon the death of Francesco, Castruccio became the governor and tutor of Pagolo, which increased enormously his power and position, and created a certain amount of envy against him in Lucca in place of the former universal goodwill, for many men suspected him of harbouring tyrannical intentions. Among these the leading man was Giorgio degli Opizi, the head of the Guelph party. This man hoped after the death of Messer Francesco to become the chief man in Lucca, but it seemed to him that Castruccio, with the great abilities which he already showed, and holding the position of governor, deprived him of his opportunity; therefore he began to sow those seeds which should rob Castruccio of his eminence. Castruccio at first treated this with scorn, but afterwards he grew alarmed, thinking that Messer Giorgio might be able to bring him into disgrace with the deputy of King Ruberto of Naples and have him driven out of Lucca.
The Lord of Pisa at that time was Uguccione of the Faggiuola of Arezzo, who being in the first place elected their captain afterwards became their lord. There resided in Paris some exiled Ghibellines from Lucca, with whom Castruccio held communications with the object of effecting their restoration by the help of Uguccione. Castruccio also brought into his plans friends from Lucca who would not endure the authority of the Opizi. Having fixed upon a plan to be followed, Castruccio cautiously fortified the tower of the Onesti, filling it with supplies and munitions of war, in order that it might stand a siege for a few days in case of need. When the night came which had been agreed upon with Uguccione, who had occupied the plain between the mountains and Pisa with many men, the signal was given, and without being observed Uguccione approached the gate of San Piero and set fire to the portcullis. Castruccio raised a great uproar within the city, calling the people to arms and forcing open the gate from his side. Uguccione entered with his men, poured through the town, and killed Messer Giorgio with all his family and many of his friends and supporters. The governor was driven out, and the government reformed according to the wishes of Uguccione, to the detriment of the city, because it was found that more than one hundred families were exiled at that time. Of those who fled, part went to Florence and part to Pistoia, which city was the headquarters of the Guelph party, and for this reason it became most hostile to Uguccione and the Lucchese.
As it now appeared to the Florentines and others of the Guelph party that the Ghibellines absorbed too much power in Tuscany, they determined to restore the exiled Guelphs to Lucca. They assembled a large army in the Val di Nievole, and seized Montecatini; from thence they marched to Montecarlo, in order to secure the free passage into Lucca. Upon this Uguccione assembled his Pisan and Lucchese forces, and with a number of German cavalry which he drew out of Lombardy, he moved against the quarters of the Florentines, who upon the appearance of the enemy withdrew from Montecarlo, and posted themselves between Montecatini and Pescia. Uguccione now took up a position near to Montecarlo, and within about two miles of the enemy, and slight skirmishes between the horse of both parties were of daily occurrence. Owing to the illness of Uguccione, the Pisans and Lucchese delayed coming to battle with the enemy. Uguccione, finding himself growing worse, went to Montecarlo to be cured, and left the command of the army in the hands of Castruccio. This change brought about the ruin of the Guelphs, who, thinking that the hostile army having lost its captain had lost its head, grew over–confident. Castruccio observed this, and allowed some days to pass in order to encourage this belief; he also showed signs of fear, and did not allow any of the munitions of the camp to be used. On the other side, the Guelphs grew more insolent the more they saw these evidences of fear, and every day they drew out in the order of battle in front of the army of Castruccio. Presently, deeming that the enemy was sufficiently emboldened, and having mastered their tactics, he decided to join battle with them. First he spoke a few words of encouragement to his soldiers, and pointed out to them the certainty of victory if they would but obey his commands. Castruccio had noticed how the enemy had placed all his best troops in the centre of the line of battle, and his less reliable men on the wings of the army; whereupon he did exactly the opposite, putting his most valiant men on the flanks, while those on whom he could not so strongly rely he moved to the centre. Observing this order of battle, he drew out of his lines and quickly came in sight of the hostile army, who, as usual, had come in their insolence to defy him. He then commanded his centre squadrons to march slowly, whilst he moved rapidly forward those on the wings. Thus, when they came into contact with the enemy, only the wings of the two armies became engaged, whilst the center battalions remained out of action, for these two portions of the line of battle were separated from each other by a long interval and thus unable to reach each other. By this expedient the more valiant part of Castruccio's men were opposed to the weaker part of the enemy's troops, and the most efficient men of the enemy were disengaged; and thus the Florentines were unable to fight with those who were arrayed opposite to them, or to give any assistance to their own flanks. So, without much difficulty, Castruccio put the enemy to flight on both flanks, and the centre battalions took to flight when they found themselves exposed to attack, without having a chance of displaying their valour. The defeat was complete, and the loss in men very heavy, there being more than ten thousand men killed with many officers and knights of the Guelph party in Tuscany, and also many princes who had come to help them, among whom were Piero, the brother of King Ruberto, and Carlo, his nephew, and Filippo, the lord of Taranto. On the part of Castruccio the loss did not amount to more than three hundred men, among whom was Francesco, the son of Uguccione, who, being young and rash, was killed in the first onset.
This victory so greatly increased the reputation of Castruccio that Uguccione conceived some jealousy and suspicion of him, because it appeared to Uguccione that this victory had given him no increase of power, but rather than diminished it. Being of this mind, he only waited for an opportunity to give effect to it. This occurred on the death of Pier Agnolo Micheli, a man of great repute and abilities in Lucca, the murderer of whom fled to the house of Castruccio for refuge. On the sergeants of the captain going to arrest the murderer, they were driven off by Castruccio, and the murderer escaped. This affair coming to the knowledge of Uguccione, who was than at Pisa, it appeared to him a proper opportunity to punish Castruccio. He therefore sent for his son Neri, who was the governor of Lucca, and commissioned him to take Castruccio prisoner at a banquet and put him to death. Castruccio, fearing no evil, went to the governor in a friendly way, was entertained at supper, and then thrown into prison. But Neri, fearing to put him to death lest the people should be incensed, kept him alive, in order to hear further from his father concerning his intentions. Ugucionne cursed the hesitation and cowardice of his son, and at once set out from Pisa to Lucca with four hundred horsemen to finish the business in his own way; but he had not yet reached the baths when the Pisans rebelled and put his deputy to death and created Count Gaddo della Gherardesca their lord. Before Uguccione reached Lucca he heard of the occurrences at Pisa, but it did not appear wise to him to turn back, lest the Lucchese with the example of Pisa before them should close their gates against him. But the Lucchese, having heard of what had happened at Pisa, availed themselves of this opportunity to demand the liberation of Castruccio, notwithstanding that Uguccione had arrived in their city. They first began to speak of it in private circles, afterwards openly in the squares and streets; then they raised a tumult, and with arms in their hands went to Uguccione and demanded that Castruccio should be set at liberty. Uguccione, fearing that worse might happen, released him from prison. Whereupon Castruccio gathered his friends around him, and with the help of the people attacked Uguccione; who, finding he had no resource but in flight, rode away with his friends to Lombardy, to the lords of Scale, where he died in poverty.