From the 1380s to the 1420s, Florence extended its authority over numerous cities and towns along the Valdichiana valley in southeast Tuscany, including three major settlements: Arezzo (1384), Montepulciano (1404), and Cortona (1411). However, the rationale behind Florence’s territorial campaign in the valley has not drawn the attention it deserves in Florentine studies. When the Florentines turned their imperialistic gaze on Valdichiana, they found that the swampy conditions along the river valley were unfavorable for settlement. The valley was also not a major traffic route in Tuscany, as the Via Francigena, a well-established pilgrimage road to Rome, was not far distant. A closer look at contemporary Italian geopolitical and diplomatic circumstances, particularly the century-long hostility between Florence and Siena, sheds light on Florence’s justifications for striving for a safer and more accessible network to the south that circumvented the territories of Siena. In addition to creating an alternate pilgrimage road leading to Rome, Florence’s attempt was most likely to provide the city greater mobility and control in the south, which was vital to the city’s rise as a dominant territorial state in Tuscany. In this sense, Valdichiana was a symbolic milestone on Florence’s path to political and military hegemony.