Siena lost its independence and ended up under the rule of Florence i.e. Tuscany because King Philip II of Spain gave it to the Medicis due to his large monetary debt.
On this day in 1555 Tuscan and Spanish forces took control of Siena, ending the centuries-old Republic of Siena. Siena and its surrounding territory had been an independent city-state since the 11th century. It possessed a significant territory, not much smaller than the neighboring Republic of Florence. Banking was one of the city’s most important economic branches.
Siena surrendered to the forces of Charles V of Habsburg, ruler of Spain and many other European countries. Siena was captured by the emperor’s general Gian Giacomo Medici, brother of future Pope Pius IV (born Giovanni Angelo Medici). Siena’s garrison had been commanded by Piero Strozzi, a big enemy of the Florentine Medicis. Siena ended up under Tuscan rule because King Philip II of Spain gave it to the Medicis due to his large monetary debt.