A Besieging Army Is Vulnerable to Outside Attack

The conduct of a siege [in the Renaissance period] was a hugely expensive undertaking in terms of supplies, consuming tons of food, gunpowder and shot. When an army was engaged upon a siege it was unable to threaten anywhere else, and also became vulnerable to outside attack. This situation is exemplified forever by the comical situation at the Siege of Turin in 1640 [C.E.]. The French in the citadel were besieged by the Spanish in the city who were in turn besieged by a French army outside the city walls who were also besieged by a Spanish army in siege lines!

— The Art of Renaissance Warfare, Chapter 8