 His most important work, "The Crisis of the Early Italian Renaissance" (1955), theorized that a threatened invasion of the Florentine city-state by Giangaleazzo Visconti of Milan had a dramatic effect on their conception of the directionality of history.Previously believing that good necessarily prevailed, upon considering the thought-to-be impending doom of the Florentine republic at the hands of Milan, some Florentine thinkers began to think otherwise.Baron theorized that it was this shift in understanding that allowed later thinkers like Niccolò Machiavelli to construct his view that free states required a politically realistic outlook in order to survive.