The Union commander at Nashville was Major General George H. Thomas (North). The kind of soldier he was matters here, because his greatest strength is exactly what nearly got him fired on the eve of his greatest day.

Thomas was a Virginian who had stayed with the Union when his state seceded, a wrenching choice that cost him his family, who never spoke to him again. On the battlefield he was famously, almost geologically, unhurried. At Chickamauga, a costly Union defeat in north Georgia in September 1863, when the rest of the Union army was routed and streaming off the field, Thomas held a hill with a stubborn rear-guard stand that saved the army from total destruction. An observer described him “standing like a rock,” and the nickname stuck: “the Rock of Chickamauga.” His men had a stack of others, “Pap” and, tellingly, “Old Slow Trot.” He did not move until he was ready. When he did, things tended to break.

At Nashville, Thomas wanted to be ready. He spent days methodically massing his force and, above all, refitting the cavalry under Brigadier General James H. Wilson (North), who was short of horses. Thomas wanted his cavalry up to strength because they were the swinging door of the whole plan he was building. To Washington, though, the delay looked like dithering with a Confederate army loose in Tennessee. Ulysses S. Grant, the Union general-in-chief, far away outside Richmond at Petersburg, became convinced that Hood might slip past Thomas entirely and drive north into the Ohio Valley while Thomas was still grooming his horses. So Grant began sending telegrams, a rising stream of them, each one sharper than the last, prodding Thomas to attack now.

Then nature itself froze Thomas in place. On December 8 a vicious ice storm hit Nashville, glazing the hills in ice and making an attack physically impossible. You cannot send tens of thousands of men charging up frozen slopes they cannot stand on. The ground stayed locked solid through December 12. Thomas could not move; Grant, hundreds of miles away, could not see the ice and grew more alarmed by the day.

This is where it nearly ended for Thomas. Grant did not just threaten, he acted. On December 13 he ordered Major General John A. Logan (North) to Nashville to take command of the army if Thomas had not yet attacked when he arrived. Logan started west and reached Louisville, a long day’s travel from Nashville, by December 15. And Grant trusted the situation so little that he did not even leave it to Logan: on December 14 Grant himself left Petersburg and started for Nashville, intending to take personal command of the whole thing, getting as far as Washington before he stopped.

So on the morning the battle began, the most decisive field victory of the entire war was about to be won by a general whose own superiors had a replacement already en route and the commanding general of all Union armies personally on a train to fire him.

And then the ice melted. Thomas wired Washington that the thaw had come and that he would attack the next morning, a calm, almost laconic message after all the panic above his head. One of his telegraph officers put the same news more colorfully, noting that the men could finally move “without skates.” Thomas attacked before the relief ever arrived, and what happened next did not merely satisfy his critics, it erased them. Grant later admitted, in so many words, that the victory at Nashville obviated all the criticism that had been heaped on Thomas. The slow man had been right. He had refused to throw his army at a problem before it was solved, and when he finally moved, he solved it completely.