Short answers, pulled from the story.
The Battle of Nashville was a two-day battle fought at Nashville, Tennessee, on the 15th and the 16th of December, 1864, between the Confederate Army of Tennessee under Lieutenant General John Bell Hood and the Union Army of the Cumberland under Major General George H. Thomas. It ended in one of the largest Union victories of the Civil War and effectively destroyed Hood's army as a fighting force.
The Battle of Nashville was fought on the 15th and the 16th of December, 1864. It followed the Battle of Franklin on the 30th of November, 1864, and the Battle of Spring Hill on the 29th of November, 1864.
The Confederate Army of Tennessee was commanded by Lieutenant General John Bell Hood, and the Union Army of the Cumberland was commanded by Major General George H. Thomas. Thomas led a combined force of about 55,000 men against Hood's roughly 30,000.
Hood retreated because Thomas routed his army over the two days of fighting, breaking the Confederate left flank at Shy's Hill on the 16th of December. The Army of Tennessee fled south toward the Tennessee River, pursued by Union cavalry until Hood crossed near Bainbridge, Alabama, by the 28th of December.
Shy's Hill was the western anchor of the Confederate line on the 16th of December, named after Colonel William M. Shy, who died defending it. Brigadier General John McArthur's division attacked it on his own initiative, and the collapse of the Confederate left there broke Hood's army.
Federal casualties totaled 387 killed, 2,562 wounded, and 112 missing. Confederate losses are harder to determine, but Thomas reported capturing 4,561 prisoners, and one historian estimated about 2,500 Confederates were killed or wounded.