The First Italian War/ Italian War of Charles VIII (1494-96) was an unsuccessful French attempt to conquer the Kingdom of Naples, and helped trigger over half a century of warfare in Italy, which ended with Spain as the dominant power (Italian Wars, 1494-1559).
The Kingdom of Naples was the mainland half of the earlier Kingdom of Sicily. Sicily had been lost to Peter III of Aragon in 1282, and the Angevin rulers of the kingdom had moved to the mainland, where they ruled until 1442 when Alfonso V of Aragon conquered the mainland, reunited the two halves of the kingdom. After his death in 1458 the kingdom was split once again. Sicily continued to be ruled by Aragon, while the mainland Kingdom of Naples went to Alfonso's illegitimate son Ferrante.
In 1494 Ferrante died, and the throne passed to his son Alfonso II of Naples. Charles VIII decided that this was the ideal opportunity to press his own claims to Naples (although preparations had been going on for some time). He also received support from Ludovico Sforza, then busily establishing himself as Duke of Milan. Alfonso had some claim to Milan himself, and this may have helped convince Ludovico to support the French.
In the spring of 1494 Charles VIII began to gather his army. He wanted 11,400 French cavalry, 6,000 Italian cavalry and 22,000 infantry, including 6,000 Swiss pikemen, and his eventual army didn’t fall much short of this. The naval element of the force began to gather at Genoa in the spring. The Neapolitans attempted to trigger an uprising at Genoa, but the attempt failed, and part of this expeditionary force was defeated on land at Rapallo on 8 September. By this point Charles had crossed the Alps and was at Asti.
Once the French began to move south they advanced rapidly. Charles entered Florence on 17 November (where he overthrew Pietro de Medici, replacing him with a constitutional republic), and Rome at the end of December. Pope Alexander VI temporarily came to terms with the French, recognising their overwhelming force.
As the French advance guard approached his borders Alfonso lost his nerve, and on 21 January 1495 he abdicated in favour of his son Ferdinand (or Ferrantino). Alfonso entered a monastery, and died in Messina on 18 December 1495.
Ferdinand attempted to defend his new kingdom. He tried to hold the line of the River Liris, but the French captured the fortress at Monte San Giovanni in a few hours, crossed the river, and threatened to outflank him. Ferdinand retreated to Capua, but his short-lived rule was undermined by the rapid progress of the French. On 22 February Charles entered Naples, and for a short period it looked as if he had achieved his aims.
Charles's rapid advance triggered the formation of an alliance against him. The League of Venice included the Emperor Maximilian, Ferdinand and Isabella of Spain, Venice and even Milan. The new league was officially a defensive alliance, but it was clear that it was aimed against Charles, and on 21 May he decided to leave Naples and return to the north of Italy. He took 7,200 French cavalry, 4,000 Swiss pikemen and 2,000 Gascon crossbowmen with him, leaving the rest of his army in Naples with Gilbert, Duke of Montpensier, his regent.
The key moment of the retreat north came in early July. The League army had taken up a position on the right bank of the River Taro at Fornovo, where they hoped to prevent Charles from crossing the Apennines. The French were badly outnumbered, but Charles managed to get his men across to the left bank of the river. This disrupted the League plans and the resulting battle of Fornovo (6 July 1495) was a narrow French victory. Both sides later claimed victory - the French because they had successfully fought their way past the League, the League because they had inflicted more casualties.
On 15 July Charles returned to Asti. He found the situation in the north of Italy had turned against him. An attempt to retake Genoa failed, and the French fleet returning from Naples was captured. Louis, Duke of Orleans, was forced to surrender at Novara. Some of his advisors wanted Charles to continue the war against Milan, but the king was uninterested in the war in the north, and he agreed a general peace with Ludovico. On 15 October Charles re-crossed the Alps. Once back in France he began to prepare for another expedition to Naples, but died in 1498 before he could return. The French throne passed to Louis of Orleans, who would soon renew the French involvement in Italy (Second Italian War/ Italian War of Louis XII (1499-1503).
In the south Ferdinand of Naples landed with a Spanish army led by Gonzalo Fernandez de Cordoba. This army suffered an initial defeat at Seminara (28 June 1495), but the French were unable to prevent Ferdinand from returning to the city by sea. He was welcomed by the citizens and the French defenders quickly withdrew. Montpensier remained at large until July 1496, when he finally surrendered, and the last French stronghold in Naples, at Gaeta, didn't fall until November. By this time Ferdinand had died - having married his half-aunt Joanna in August the new couple moved to Somma-Vesuvius, where Ferdinand died of an illness on 7 September 1496. The crown passed to Frederick IV, the brother of Alfonso II.
Frederick didn't enjoy his throne for long. In 1500 Ferdinand of Aragon and Louis XII of France made a secret agreement to divide Naples between them. Frederick was very quickly deposed (Second Italian War/ Italian War of Louis XII, 1499-1503), but the temporary allies soon fell out and the main fighting was between France and Spain.