The Guelphs and Ghibellines (/ˈɡwɛlfs ... ˈɡɪbɪlaɪnz/ GWELFS ... GHIB-il-ynze, US also /-liːnz, -lɪnz/ -⁠eenz, -⁠inz; Italian: guelfi e ghibellini [ˈɡwɛlfi e gɡibelˈliːni, -fj e -]) were factions supporting the pope (Guelphs) and the Holy Roman emperor (Ghibellines) in the Italian city-states of Central and Northern Italy during the Middle Ages. During the 12th and 13th centuries, rivalry between these two parties dominated political life across Italy. The struggle for power between the Papacy and the Holy Roman Empire arose with the Investiture Controversy, which began in 1075 and ended with the Concordat of Worms in 1122.

The conflict between Guelphs and Ghibellines arose from the political divisions caused by the Investiture Controversy, about whether secular rulers or the pope had the authority to appoint bishops and abbots. Upon the death of Emperor Henry V, of the Salian dynasty, the dukes elected an opponent of his dynasty, Lothair III, as the new emperor. This displeased the house of Hohenstaufen, who were allied with and related to the old dynasty. Out of fear of the Hohenstaufen, Lothair III placed himself under the pope's protection. To this end, he ceded all Imperial rights to the pope under Henry V's Concordat of Worms. War then broke out in Germany between those who supported the Hohenstaufen, and those who were aligned to Lothair and the pope. Upon Lothair's death, the Hohenstaufen Conrad III was elected, while Lothair's heir, Henry the Proud, of the House of Welf, continued fighting.

Guelph (often spelled Guelf; in Italian Guelfo, plural Guelfi) is an Italian form of the name of the House of Welf, the family of the dukes of Bavaria (including the namesake Duke Welf II of Bavaria, as well as Henry the Lion). The Welfs were said to have used the name as a rallying cry during the Siege of Weinsberg in 1140, in which the rival Hohenstaufens (led by Conrad III) used "Wibellingen" (the name of a castle today known as Waiblingen, as their cry; "Wibellingen" subsequently became Ghibellino in Italian). Thus, the Hohenstaufen faction became known as the Ghibellines and the Welfs eventually became known as the Guelphs. The Ghibellines were the imperial party, while the Guelphs supported the pope. Cities more directly threatened by the enlargement of the Papal States tended to align with the Ghibelline faction, while the cities that wanted more autonomy from the Empire tended to belong to the Guelph faction. The clash between the municipalities of Northern Italy and imperial power originated in the struggle for investitures. The Guelph Lombard League defeated Emperor Frederick Barbarossa at the Battle of Legnano in 1176. Frederick recognized the full autonomy of the cities of the Lombard league under his nominal suzerainty.

The conflict between the two factions dominated the politics of medieval Italy, and persisted long after the confrontation between emperor and pope had ceased. Smaller cities tended to be Ghibelline if the larger city nearby was Guelph. For example, Guelph Republic of Florence and Ghibelline Republic of Siena faced off at the Battle of Montaperti, 1260. Pisa maintained a staunch Ghibelline stance against her fiercest rivals, the Guelph Republic of Genoa and Florence. Local or regional political reasons motivated political alignments. Within cities, party allegiances differed from guild to guild, rione to rione, and a city could easily change party after internal upheavals. The conflicts between Guelphs and Ghibellines ended in the 14th century with the creation of a new situation, where the State and the laity began to withdraw from any ecclesiastical interference.

At the beginning of the 13th century, Philip of Swabia, a Hohenstaufen, and Otto of Brunswick, a Welf, were rivals for the imperial throne. Philip was supported by the Ghibellines as a son of Frederick I, while Otto was supported by the Guelphs. Although the Guelphs initially succeeded in getting Otto crowned as Emperor, Otto turned against the Papacy, was excommunicated, and was replaced with Philip's heir Emperor Frederick II. Frederick II was an enemy of both Otto and the papacy, and during Frederick's reign, the Guelphs became more strictly associated with the papacy while the Ghibellines became supporters of the Empire and Frederick in particular. Pope Gregory IX excommunicated Frederick II in 1227 for failing to go on Crusade, then again for going on the Sixth Crusade (1228–1229) while excommunicated. While Frederick was in the Crusader states, this division developed there, and his regent in Italy fought a war with the Pope. That war was ended and the excommunication was lifted in 1230, but the hostility continued.

In 1237, Frederick entered Italy with a large army, intending to subdue the defiant cities of the Lombard League. Pope Gregory tried to broker a peace, but failed. Frederick defeated the League at Cortenuova and refused all peace offers from them. He besieged Brescia but was repulsed. In 1239, Frederick was again excommunicated by Pope Gregory. In response, he expelled Franciscan and the Dominican friars from Lombardy and made his son Enzo Imperial vicar in Italy. He also annexed Romagna, Marche, the Duchy of Spoleto, and part of the Papal States, and marched through Tuscany hoping to capture Rome. He was forced to retreat, sacking the city of Benevento. Soon the Ghibelline city of Ferrara fell and Frederick once more advanced, capturing Ravenna and Faenza. The Pope convened a council, but an Imperial-Pisan fleet defeated a Papal fleet carrying cardinals and prelates from Genoa in the Battle of Giglio. Frederick approached Rome. Meanwhile, Pope Gregory died. Frederick withdrew his forces and freed two cardinals he had jailed in Capua. However, Frederick marched again against Rome throughout 1242 and 1243.

A new pope – Innocent IV – was elected. At first, Frederick was pleased with the election since Innocent had relatives in the Imperial camp. However, the new Pope immediately turned against Frederick. When the City of Viterbo rebelled, the Pope backed the Guelphs. and Frederick immediately marched to Italy and besieged Viterbo. The pope signed a peace treaty with the emperor, relieving the city. After the Emperor left, Cardinal Raniero Capocci, leader of Viterbo, had the garrison massacred. The Pope made another treaty but he immediately broke it and continued to back the Guelphs. The Pope supported Henry Raspe, Landgrave of Thuringia as King of the Romans and soon plotted to have Frederick killed. After an attempted assassination failed, the pope fled to Liguria. Soon the tide turned against the imperial party. The Lombard city of Parma rebelled. Enzo – who had not been present – asked his father for help. Frederick and Ezzelino III da Romano, the Tyrant of Verona, laid siege to the city. The imperial camp was ambushed by the Guelphs and in the ensuing Battle of Parma the imperial party was routed, losing much of their treasury.

Frederick lost his momentum against the rebellious communes in the immediate future. Sensing this, Innocent began plans for a crusade against Sicily. Frederick soon recovered and rebuilt an army, but this defeat encouraged resistance in many cities that could no longer bear the fiscal burden of his regime: parts of the Romagna, Marche and Spoleto were lost. In May 1248, Frederick's illegitimate son Richard of Chieti defeated a papal army led by Hugo Novellus near Civitanova Marche and recaptured some areas of the Marche and Spoleto. Basing himself in Piedmont in June, Frederick hosted many nobles of northern Italy and ambassadors from foreign kings in his court, and his deposition, it seems, had not diminished his fame or preeminence. Nevertheless, it was only by strenuous, even unrelenting effort that Frederick was able to stabilize the situation by the close of 1248 and replenish his coffers, raising some 130,000 gold ounces. Frederick remained confident but after several years of war and conspiracy, he was increasingly suspicious and wearied.