Money basically. It does not have a direct relation to the warfare aspect but you can compare it to the rise of the middle class in modern times. The renaissance was a re-birth of art and science in Italy, so because people had new discoveries, inventions and ideas coming at them from every side, they had the spark of civilization ignited and while the feudal were easing there imperialistic/papal hold, the poor were eagerly meeting them half way in demanding better standards of living. And you have to consider the fact that with merchants, it not just businessmen that come up. These nouveau riche will demand a whole host of services like accountants and bankers, doctors and lawyers and teachers; and a whole cycle is put into motion that has become the predominant force of social transformation. There was open warfare, the French had wrested quite some area from the Rome-based papacy but they soon lost it due to negligence of local issues and not being close to the newly won over people. It should be noted that, foreigners were invited into Italy by local dukes and princes to avoid the take over of their fiefdoms by other Italian rivals.