Before long the reciprocal need of fresh territory and frontier disputes, especially concerning Poggibonsi and Montepulciano, led to an outbreak of hostilities between Florence and Siena. Thereupon, to spite the rival republic, the Sienese took the Ghibelline side, and the German emperors, beginning with Frederick Barbarossa, rewarded their fidelity by the grant of various privileges.
During the 12th and 13th centuries there were continued disturbances, petty wars, and hasty reconciliations between Florence and Siena, until in 1254–1255 a more binding peace and alliance was concluded. But this treaty, in spite of its apparent stability, led in a few years to a fiercer struggle; for in 1258 the Florentines complained that Siena had infringed its terms by giving refuge to the Ghibellines they had expelled, and on the refusal of the Sienese to yield to these just remonstrances both states made extensive preparations for war. Siena applied to Manfred, obtained from him a strong body of German horse, under the command of Count Giordano, and likewise sought the aid of its Ghibelline allies. Florence equipped a powerful citizen army, of which the original registers are still preserved in the volume entitled Il Libro di Montaperti in the Florence archives. This army, led by the podesta. of Florence and twelve burgher captains, set forth gaily on its march towards the enemy's territories in the middle of April 1260, and during its first campaign, ending on the 18th of May, won an insignificant victory at Santa Petronilla, outside the walls of Siena. But in a second and more important campaign, in which the militia of the other Guelf towns of Tuscany took part, the Florentines were signally defeated at Montaperti on the 4th of September 1260. This defeat crushed the power of Florence for many years, reduced the city to desolation, and apparently annihilated the Florentine Guelfs. But the battle of Benevento (1266) and the establishment of the dynasty of Charles of Anjou on the Neapolitan throne put an end to the Ghibelline predominance in Tuscany. Ghibelline Siena soon felt the effects of the change in the defeat of its army at Colle di Valdelsa (1269) by the united forces of the Guelf exiles, Florentines and French, and the death in that battle of her powerful citizen Provenzano Salvani (mentioned by Dante), who had been the leading spirit of the government at the time of the victory of Montaperti. For some time Siena remained faithful to the Ghibelline cause; nevertheless Guelf and democratic sentiments began to make head. The Ghibellines were on several occasions expelled from the city, and, even when a temporary reconciliation of the two parties allowed them to return, they failed to regain their former influence.
Meanwhile the popular party acquired increasing power in the state. Exasperated by the tyranny of the Salimbeni and other patrician families allied to the Ghibellines, it decreed in 1277 the exclusion of all nobles from the supreme magistracy (consisting since 1270 of thirty-six instead of twenty-four members), and insisted that this council should be formed solely of Guelf traders and men of the middle class. This constitution was confirmed in 1280 by the reduction of the supreme magistracy to fifteen members, all of the humbler classes, and was definitively sanctioned in 1285 (and 1287) by the institution of the magistracy of nine. This council of nine, composed only of burghers, carried on the government for about seventy years, and its rule was sagacious and peaceful. The territories of the state were enlarged; a friendly alliance was maintained with Florence; trade flourished; in 1321 the university was founded, or rather revived, by the introduction of Bolognese scholars; the principal buildings now adorning the town were begun; and the charitable institutions, which are the pride of modern Siena, increased and prospered. But meanwhile the exclusiveness of the single class of citizens from whose ranks the chief magistrates were drawn had converted the government into a close oligarchy and excited the hatred of every other class. Nobles, judges, notaries and populace rose in frequent revolt, while the nine defended their state (1295–1309) by a strong body of citizen militia divided into terzieri (sections) and contrade (wards), and violently repressed these attempts. But in 1355 the arrival of Charles IV. in Siena gave fresh courage to the malcontents, who, backed by the imperial authority, overthrew the government of the nine and substituted a magistracy of twelve drawn from the lowest class. These new rulers were to some extent under the influence of the nobles who had fomented the rebellion, but the latter were again soon excluded from all share in the government.
This was the beginning of a determined struggle for supremacy, carried on for many years, between the different classes of citizens, locally termed ordini or monti—the lower classes striving to grasp the reins of government, the higher classes already in office striving to keep all power in their own hands, or to divide it in proportion to the relative strength of each monte. As this struggle is of too complex a nature to be described in detail, we must limit ourselves to a summary of its leading episodes.
The twelve who replaced the council of nine (as these had previously replaced the council of the nobles) consisted—both as individuals and as a party—of ignorant, incapable, turbulent men, who could neither rule the state with firmness nor confer prosperity on the republic. They speedily broke with the nobles, for whose manoeuvres they had at first been useful tools, and then split into two factions, one siding with the Tolomei, the other, the more restless and violent, with the Salimbeni and the noveschi (partisans of the nine), who, having still some influence in the city, probably fomented these dissensions, and, as we shall see later on, skilfully availed themselves of every chance likely to restore them to power. In 1368 the adversaries of the twelve succeeded in driving them by force from the public palace, and substituting a government of thirteen—ten nobles and three noveschi.
This government lasted only twenty-two days, from the 2nd to the 24th September, and was easily overturned by the dominant faction of the dodicini (partisans of the twelve), aided by the Salimbeni and the populace, and favoured by the emperor Charles IV. The nobles were worsted, being driven from the city as well as from power; but the absolute rule of the twelve was brought to an end, and right of participation in the government was extended to another class of citizens. For, on the expulsion of the thirteen from the palace, a council of 124 plebeians created a new magistracy of twelve difensori (defenders), no longer drawn exclusively from the order of the twelve, but composed of five of the popolo minuto, or lowest populace (now first admitted to the government), four of the twelve, and three of the nine. But it was of short duration, for the dodicini were ill satisfied with their share, and in December of the same year (1368) joined with the popolo minuto in an attempt to expel the three noveschi from the palace. But the new popular order, which had already asserted its predominance in the council of the riformatori, now drove out the dodicini, and for five days (11th to 16th December) kept the government in its own hands. Then, however, moved by fear of the emperor, who had passed through Siena two months before on his way to Rome, and who was about to halt there on his return, it tried to conciliate its foes by creating a fresh council of 150 riformatori, who replaced the twelve defenders by a new supreme magistracy of fifteen, consisting of eight popolani, four dodicini, and three noveschi, entitled respectively " people of the greater number," " people of the middle number," and " people of the less number." From this renewal dates the formation of the new order or monte dei riformatori, the title henceforth bestowed on all citizens, of both the less and the greater people, who had reformed the government and begun to participate in it in 1368. The turbulent action of the twelve and the Salimbeni, being dissatisfied with these changes, speedily rose against the new government. This time they were actively aided by Charles IV., who, having returned from Rome, sent his militia, commanded by the imperial vicar Malatesta da Rimini, to attack the public palace. But the Sienese people, being called to arms by the council of fifteen, made a most determined resistance, routed the imperial troops, captured the standard, and confined the emperor in the Salimbeni palace. Thereupon Charles came to terms with the government, granted it an imperial patent, and left the city, consoled for his humiliation by the gift of a large sum of money.