Sforza is the other, non-Borgia side of Machiavelli's discussion about men who come to power either through luck or hard work. It's okay if you don't remember him too much, because Machiavelli says one sentence about him before commencing his love affair with Borgia. Still it's good to know who he is.
As Machiavelli says, "With the right policies and great courage, Sforza, a commoner, became Duke of Milan and, having won power with enormous effort, held on to it easily enough" (7.3). He was the son of a Mercenary leader and started fighting alongside his dad when he was eighteen. Soon he got a reputation for begin able to bend metal bars with his hands (what?!) and became famous.
One of the people who paid Sforza to fight for him was Filippo Maria Visconti, the Duke of Milan. Since Sforza married his daughter, he expected to become duke when Visconti died in 1447. Not so fast, Sforza. Milan decided it wants to be a republic. Not so fast, Milan. What Sforza wants, Sforza gets. When Milan hired him to fight against Venice, he betrayed them and instead took Milan in 1450.
Oops. That is why you shouldn't use mercenaries.
Machiavelli also uses the swift decline of Sforza's kingdom to illustrate that rulers should always have war on their minds. He says, "A military man with his own army, Francesco Sforza rose from commoner to Duke of Milan; shunning military hardships, his sons fell from dukes to commoners" (14.2)—right back to where they started. Thus ends the story of the most successful and famous mercenary in Europe.