Why did Savannah surrender to William Tecumseh Sherman without a siege?

Savannah did not surrender to William Tecumseh Sherman because of a formal siege; rather, the city's position became untenable, leading the Confederate garrison to evacuate.

After Sherman’s forces surrounded the city on land and successfully captured Fort McCallister—a vital component of the city's southern defenses—the Confederate commander, Lieut. Gen. William J. Hardee, recognized that his situation was unsustainable. Although Hardee initially rejected Sherman’s demand for surrender on December 17, 1864, he chose to abandon the city's trenches on the night of December 20–21. His troops retreated across the Savannah River into South Carolina, allowing the mayor of Savannah to formally surrender the city to Union forces on December 21, 1864.12