The Great Venetian War (1488-1498) is usually referred to as the opening phase of the chaos which engulfed Italia during the 16th century. It incorporates various smaller wars and campaigns and was barely ever referred to by its current name until the mid-1600s. It is often thought that its name refers to the intent behind the war rather than its length or achievements and is sometimes read as a sarcastic commentary on Luxembourg policy during the 15th and 16th centuries.
Many place the blame for the war squarely at Emperor Sigismund II's door. He, through his Hungarian connections, clung to a claim to the Neapolitan throne which had been violently usurped by the Aragonese Barcelona dynasty over the course of the Aragonese Conquest of Naples. His predecessors had plowed their diplomatic and military energies into the reconquest of the kingdom, almost at the cost of the rest of their possessions but had made little headway. Much of their failure had been caused by the alliance at either end of the Adriatic; Venice and Byzantium, both extremely opposed to having the sea turned into a 'Hungarian Lake'. Together they had rebuffed concerted attacks by Hungary in the 1460's. Realising the alliance would most likely block another putative conquest Sigismund instead tried to isolate them, choosing to focus efforts on reducing Venice's reach before Byzantium could potentially bring its lumbering armies into play in the Balkans.
However, there were other forces at play. In Italia itself the Duchy of Milan had been in an on-off conflict with Venice which had led to loss of territory to its republican neighbour, meanwhile it had fallen into a succession crisis between the illegitimate sons of Matteo IV Visconti, themselves backed by France, and Henry IV of Castile who claimed it via his Visconti mother. Aragon too had not given up the claim to Naples and John IV wished to start his reign by wresting it from his Neapolitan cousin James III. Finally, the Genoese Pope, Paul III, as well as supporting the Luxembourg claim to Naples, had a deep seated hatred of Venice and had put a great deal of effort into opposing its slow accretion of the North Italian mainland. Therefore when Sigismund II came in 1488 to forge a grand alliance against Venice, the 'Holy League', he found numerous parties willing to get involved and grab a portion of Italia for themselves.
In August 1488 Venice seized the Duchy of Fruli, the last remaining part of the Patriarchy of Aquelia and a battleground for Venetian and Hungarian influences for several decades. The seizure was inevitably declared illegal by Sigismund II and Pope Paul III, and Imperial, Hungarian, Genoese and Papal forces were soon bearing down on the Serene Republic. In Milan the Viscontis rallied their forces to Venice's aid while their main backer, France, quickly scrambled to plant a major force in the duchy (which would lead to its own problems). Castile would eventually get a force to Milan (alongside Swiss mercenaries) to back Henry IV's claims but only at the cost of letting a considerable Aragonese force into the duchy too. The grand anti-Venice alliance scored significant victories over Venice at Adda River and Pandino. However, the entry of Castile and Aragon into the peninsula made the others waver in their loyalties and the League failed to capture any significant Venetian fortresses.
To the east Byzantium had raised an army in Albania much quicker than Sigismund had bargained for. Whilst Bohemian and other German troops remained in play in the Po Valley Sigismund's Hungarian forces were withdrawn to deal with the Byzantines. The so-called 'Travunijan War' saw a large but inconclusive clash at Trebinje followed by a heavy Hungarian defeat at Kosovo Field in October 1491, although in the end the sideshow achieved little but the ravaging of Montenegro and Kosovo, and mostly further undermined Serbia's already fragile independence.France had raised a considerable army in support of the Visconti and Venetians but had no means of getting it to Milan, the southern routes were blocked, as always, by Auvergne and Imperial roads through the Rhineland and the Swiss Confederation were out of the question. Attacking Castile directly was a non-starter as Castile had the larger navy and Aquitaine maintained good relations with its southern neighbour. Henry II therefore struck out at the Duchy of Luxembourg, diverting more German soldiers away from Italia. Sigismund played on Henry II's tenuous claims to the French crown and bankrolled Auvergne's 'Succession War' to distract them. This attempt to turn France into a pro-Luxembourg state failed as Auvergne's initially impressive cavalry advance was blunted by French, and Danish, infantry before they could reach Paris but would lessen the pressure on Luxembourg itself.
Meanwhile the Holy League's membership was changing. Paul III had died in early 1491 and was replaced with the Pisan Pius III. Pius III was much more concerned with the growth of foreign influence in Italia than Venice (and had commercial interests over the salt trade which the Visconti's were busy disrupting) and therefore promptly swapped to join Venice's side opposing Castile and Aragon, flooding Milan with mercenaries from the Swiss territories, as well as the grand Manx and Icelandic mercenary companies. The Castilian army had been thoroughly defeated in 1490 whereupon it retreated to Genoa and promptly disbanded despite Henry IV's best efforts to hold it together. Aragonese involvement had quickly changed from to actively challenging Venice to trying to seize a foothold on the peninsula for itself.
Although the war carried on much the same without him, Sigismund's direct involvement waned in 1493. Not only had Hungary risen in revolt, a partial result of his heavy-handed rule there which needed the Bohemian forces to quash it, but he had embarked on another venture to seize the Anglian throne from Eric IX of Denmark. Eventually in 1494 the war shuddered to a unsatisfactory conclusion. The 'Holy League', by this point effectively only Aragon, had seized Sarzana in Genoa which was enough for John IV, at least for now. The collapse of Castile's army in Milan ended the succession crisis and led to an understanding between Milan and Venice over their territory. The Treaty of Bassano, signed in March 1494 halted direct attacks on Venice and would allow them breathing space to plow money and resources into creating a real army for the republic. Sigismund would sign his own treaty with the papacy, making sure future popes rejected Aragonese claims on Naples and making the Luxembourg claim official.
The entry of several European armies into Italia, plus seemingly endless mercenaries would not stop the peninsula erupting into violence again, though foreign involvement simmered while parties were distracted by the War of Anglian Succession. Sigismund's son John III would, in time, call upon the papacy to excommunicate Naples, beginning another general war, but in the meantime the city-states called on the vast wealth and manpower available to plunge into the vicious Mantuan Wars.