The Battle of Monocacy, which took place four miles south of Frederick, Maryland on a blisteringly hot day in 1864, was a full-field engagement between some 12,000 battle-hardened Confederate troops led by the controversial Jubal Anderson Early, and some 5,800 Union troops, many of them untested in battle, under the mercurial Lew Wallace. When the fighting ended, Early had routed Wallace in the northernmost Confederate victory of the war.

Two days later, on another brutally hot afternoon, the foul-mouthed, hard-drinking Early sat astride his horse outside the gates of Fort Stevens in the upper northwestern fringe of Washington, D.C. He was about to make one of the war's most fateful, portentous decisions: whether or not to order his men to invade the nation's capital.

Once manned by tens of thousands of experienced troops, Washington's ring of forts and fortifications that day were in the hands of a ragtag collection of walking wounded Union soldiers, the Veteran Reserve Corps, along with what were known as hundred days' men---raw recruits who had joined the Union Army to serve as temporary, rear-echelon troops. It was with great shock, then, that the city received news of the impending rebel attack. With near panic filling the streets, Union leaders scrambled to coordinate a force of volunteers.

But Early did not pull the trigger. With his men exhausted after the fight at Monocacy and the ensuing march, Early paused before attacking the feebly manned Fort Stevens, giving Union General Ulysses Grant just enough time to send thousands of veteran troops up from Richmond. In the battle that followed, Abraham Lincoln became the only sitting president in American history to come so close to military action that he was fired upon by the enemy.

Historian Marc Leepson shows that had Early arrived in Washington one day earlier, the ensuing havoc easily could have brought about a different conclusion to the war. He uses a vast amount of primary material, including memoirs, official records, newspaper accounts, diary entries and eyewitness reports in a reader-friendly and engaging description of the events surrounding what became known as "the Battle That Saved Washington."