Fought in the final years of the 20-year long war, the campaign saw the alliance make some territorial gains on the island before ultimately withdrawing in the face of Sicilian resistance.

[2] To end the stalemate, Pope Boniface VIII conducted an extensive diplomatic offensive to win the Kingdom of Aragon, Sicily's ally from the start of the war, to the Angevin's side, hoping a newly formed Angevin–Aragonese–Papal alliance would be able to force the Sicilians to submit.

In Rome, James negotiated a treaty in which he agreed to make war on his brother and Sicily in exchange for compensation, namely money and a papal sanction to annex Sardinia and Corsica.

Aragon possessed a formidable navy led by the famous Aragonese–Sicilian admiral Roger of Lauria, who was himself a former subject of the Kingdom of Sicily, and Angevin Naples had a large, well-equipped army trained in the style of medieval France.

With Syracuse besieged, the allies conducted a limited push inland, capturing the towns of Sortino, Ferla, Palazzolo and Buscemi before an attempt to seize Buccheri was thwarted by armed townspeople.

[2] Frustrated with the state of the siege and the ongoing Sicilian resistance, the allies looked for ways to gain control over the towns and castles in the East-Central Sicily, hoping to cut the island in two.

During the winter of 1298 a Sicilian noble named Giovanni di Barresi attempted to defect to the allied cause, promising to turn over control of his lands in Capo d’Orlando and Pietraperzia—a blow that would effectively allow James to combine his forces in northern and southern Sicily.

The Aragonese king extended a tentative peace offer to his brother, suggesting that he would withdraw his forces from the theatre in exchange for the 16 ships and prisoners the Sicilians had taken at Messina.

Frederick ultimately chose to attack his brother's battered force as it withdrew, but a storm covered the allied retreat and prevented a major fleet action.

In the ensuing Battle of Cape Orlando on 4 July, the Sicilian fleet suffered a major defeat, losing 18 galleys and granting the allies command of the sea.

From their landing site at San Marco d'Alunzio, an allied army under Roger of Lauria and Robert of Naples moved south towards the north slopes of Mount Etna, planning on cutting through the country all the way to Catania on the southern coast.

[6] The loss of Catania was a heavy blow to the Sicilian kingdom, as the allies now possessed a major port in southern Sicily and a landing site in the north.

Though the Sicilians still controlled large parts of the countryside, a string of allied-controlled towns and castles in the interior threatened to cut Frederick off from his capital in Palermo and primary power base in western Sicily.

Frederick's new position in the central Sicilian highlands moved him away from the larger costal cities, but also strengthened his internal lines of communication, as from Enna's commanding plateau he was able to send out forces to counter the allies wherever they chose to attack.

To accomplish this plan, in November 1299 an invasion force led by Robert's brother, Prince Philip of Taranto, landed on the northwest coast of Sicily and besieged Trapani.

The loss of the western army effectively ended the allied campaign for the year, with Roger of Lauria sailing back to Naples to collect reinforcements.

As part of the ploy, a castellan convinced a prisoner (who had been captured at Falconaria in December) that he wished to defect to the allied cause, citing an alleged desire to remain loyal to the pope.

The noble was then allowed to leak news of the alleged defection to Robert, to whom the defector offered to open the gates of Gagliano, a strategically important castle less than 30 miles from Frederick's headquarters in Enna.

Despite being strongly defended, the city – fatigued after years of war – was surrounded by a ring of Angevin-controlled castles to the west, Catania to the south, and had no fleet to protect it.

Seeing that the veteran Sicilian army would now be able to attack him from the high ground around Messina, Robert chose to withdraw his main force from the outskirts of the city, sailing back to Catania while leaving some men behind to garrison the allied-controlled castles in the area.

[7] The Angevins also began to suffer from lack of food, as the scorched earth campaign they had conducted to starve Messina had also destroyed the farmland the allies used to feed their army and garrisons.

Seeking to break the allied siege, he conducted an ambitious overland march through the foothills west of Messina, bypassing allied-held castles and successfully reaching the city with supplies.

The king's presence in Messina bolstered the defender's morale, and during his withdraw Frederick's army evacuated the city's women and children, allowing the garrison to further stretch out its food supply.

While the allies still maintained control over parts of the north coast of Sicily and Catania, the failure of another push inland convinced Robert to seek a peace accord with Frederick.

[7] In 1302, Prince Charles of Valois (a veteran of an earlier phase of the War of the Sicilian Vespers, in which France had fought against Aragon in Iberia) marched into northern Italy at the head of an army of feudal levies and mercenaries.

Acting as a prince of France, but officially independent of the French crown, Charles was seeking to acquire lands he had legal claim to, as in 1301 he had been declared the titular Emperor of Constantinople through his marriage to Catherine de Courtenay.

Receiving significant financial backing from the papacy and his allies in the French nobility, he used his army to crush supporters of the anti-papal Ghibellines in Tuscany and Florence before moving south.

[7][1] Arriving in Naples, Valois signed an accord with the pope and Angevins pledging to conquer Sicily in their name in return for their support in his forthcoming invasion of Byzantium and Greece.

While Roger of Lauria raided the coastline near Palermo, the Valois army waited for resupply, giving Frederick time to muster his forces at Polizzi, 20 miles away.

Cut off from his beachhead in Tremini and surrounded by barren, hostile countryside, Valois elected to march southwest to Sciacca, where he planned to link up with the allied fleet and resupply by sea.