Pazzi Conspiracy
Allies of the Conspirators
The conspirators already had a number of major players, including the pope himself. In addition, they had the support—the hidden support—of two others.
King Ferrante of Naples
King of Naples
Ferrante was an illegitimate son who had fought his way to the throne of Naples and was now one of the great rulers in Italy. His court was grand, and he had ambitions to the north. Florence was one of his most serious obstacles, and if he could help someone who was willing to undermine the city, why that could only redound to the benefit of Naples. He had no personal grudge; it was just business.
Federigo da Montefeltro, Duke of Urbino
Duke of Urbino
Federigo was strictly an opportunist in this scenario. He was a conditierre himself, hiring out at princely prices and building quite a spectacular court for himself at Urbino. He hoped to gain some specific territories out of this deal, and all he had to do was dispatch some troops at a key moment. Easy money.
Allies of the Medici
Milan was Florence's great ally. The alliance had been forged by grandpa Cosimo and the great condotierre Francesco Sforza, via a personal friendship. This had persisted through Piero's reign in Florence and Galeazzo Maria in Milan. Ruling there now was Gian Galeazzo Sforza and the alliance was a little frayed, but still strong. The personal connection wasn't there, but they still shared common enemies and no longer had a desire to infringe on one another's territories.