What kind of alliances should a prince make?

From the text:

A Prince ought to take care never to make an alliance with one more powerful than himself for the purpose of attacking others, unless necessity compels him, as is said above; because if he conquers you are at his discretion, and princes ought to avoid as much as possible being at the discretion of any one. The Venetians joined with France against the Duke of Milan, and this alliance, which caused their ruin, could have been avoided.