And where would the men to form a huge army come from? How would the men be paid? And most importantly, how could one ruler stop all the other states from ganging up on them once they realized they wanted to conquer them?
It's a bit incorrect to refer to the Italian States of the Renaissance as "City-States." Although they were very often the result of single cities dominating their neighbors between the year 1000 up until their solidification after 1300 (more or less) they were for all intents and purposes early modern polities with well-defined borders. In 1405, for example, the territories of the Republic of Venice included the important cities of Treviso, Padua, Vicenza, and Verona, each with their own history, heritage, and proud citizenry. Indeed, the cities of Padua and Verona housed a university, while the capital of the republic, Venice, did not! The Duchy of Milan was also much more than the city of Milan and its surroundings, with a large University in Pavia, and important urban centers like Lodi, Cremona, and Como.
Some rulers were able to build up large, powerful states: Can Francesco della Scala, popularly called Canagrande, built a large state around his capital of Verona. In fact, by 1329 the poet Niccolò de' Rossi affirmed that Canagrande "Will be king of Italy by next year." Canagrande instead dropped dead at forty and did no such thing. Canagrande's successor, his brother Mastino della Scala, was defeated by a Venetian-Florentine-Milanese coalition in 1339.
In the decades following the Veronese defeat, Gian Galeazzo Visconti, the Lord of Milan, was able to construct a powerful state in northern Italy. However, when Gian Galeazzo died in 1402 his widow Valentina Visconti (yes, they were first cousins) was unable to stop surrounding rulers from chiseling away her domain.
Valentina even proposed Venetian tutelage over a Veronese-Vicentine buffer state which would separate the Lombard heartland from the voracious lang-grabbing of the Carrara, rulers of Padua, foreseeing a protracted conflict. The Vicentines, for their part, unenthusiastic about becoming the designated trampling ground for a three-way power struggle between Milan, Venice, and Padua, sent a delegation to Venice and orchestrated the city's adhesion to the Republic (the Veronese opinion on the matter wasn't recorded; Francesco da Carrara had made a beeline for Verona as soon as Gian Galeazzo dropped dead, while the Vicentines got lucky that the residual force laying siege to Vicenza wasn't large enough to completely cut the city off). Venice hastily managing to assemble a ceremony welcomed Vicenza's submission. Once the ceremony was over, the Venetian Doge and Senate laid pen to paper to a letter elaborating how aghast they were that Francesco da Carrara would lay siege to one of their cities, and as a consequence they were declaring war on Padua.
By 1405, the Republic of Venice liberated Verona from the Paduan occupiers; the people of Verona rejoiced and before the city could be returned to the Visconti the town's council sent a delegation to Venice proclaiming they too wanted to adhere to the Republic. Traveling in much more security and much greater numbers than the Vicentines (the Carrara had been hounded back to Padua, where they were under siege) an absurdly pompous day-long ceremony then took place, culminating with the Doge's speech paraphrasing the words of the Prophet Isaiah: "The people that walked in darkness have seen a great light."
In 1406, with the Venetian army occupying Padua and the Carrara chased away, it was then the turn of a Paduan delegation to participate in an even greater and more splendid day-long ceremony, tournament, and feast at the end of which the banner of Padua and the key of the city was traded for the Venetian Banner of St. Mark embodied in gold because it appears the Venetians were making a sport over how absurdly elaborate they could make these ceremonies. No peace treaty was even signed between the Doge and Francesco da Carrara.
So this meant that in the 15th century (right when the Renaissance is in full swing) Northern Italy is dominated by two evenly matched powers: the Duchy of Milan and the Republic of Venice.
The Republic of Venice, although weighed down by unwieldily bureaucracy, was the wealthier of the two powers. The decision-making process in the Duchy of Milan was more streamlined, however, it was mitigated by the individual shortcomings of whichever Duke was in power. Both states intervened in conflicts involving their smaller neighbors, resulting in three conflicts in a wider Milanese-Venetian power struggle.
