The Greek historian Thucydides suggested that when a state falls as a result of a decisive battle, we should inquire not into the battle itself but into the underlying weakness of a state vulnerable to such a hazard of war. France in 1940 comes to mind, or Alexander’s victory over the Persians at Gaugamela, or the triumph of Wellington and Blucher at Waterloo. But such battles are rare. Rome recovered from successive defeats at the hands of Hannibal, and Hitler’s Reich, imperial Japan, Napoleon’s empire, and France during the reign of Louis XIV were able to fight on after repeated military disasters.

The Battle of Gettysburg, which was fought in the first three days of July 1863, was not the decisive clashof the Civil War. When Robert E. Lee lost the battle, in the same week that Vicksburg fell to Ulysses S. Grant (who thus cut the Confederacy in two by winning control of the Mississippi River), the South did not collapse. It fought on, battered and invaded, for almost two years, until Lee surrendered in April 1865 at Appomattox Courthouse.

And yet it is to the bloody encounters at Seminary Ridge, Little Round Top, and Devil’s Den, the Wheatfield and Cemetery Ridge, that historians return in extraordinary numbers. A comprehensive bibliography in 2004 counted 6,193 books, articles, essays, and pamphlets on the Gettysburg campaign, and many more on specific days, events, or individuals. So why, other than the marketing power of this year’s 150th anniversary, the need for yet another volume?

The short answer is that Allen C. Guelzo, who directs the Civil War Era Studies program at Gettysburg College, is supremely qualified to write it. He is an eminent historian of slavery and of the Lincoln-Douglas debates, and author of a fine biography of Lincoln. He knows the ground—all 6,000 acres of today’s battlefield park and its 1,324 monuments, statues, and plaques. Above all, he has read widely and thought deeply about the battle, not simply as a military encounter but as a political episode; hence his subtitle, “The Last Invasion.” This is the finest single-volume account available, and one that illustrates the essential paradox of the event.

“War is not so much a matter of arms as of money,” Thucydides noted. And when we count the North’s advantages in money, industrial potential, manpower, naval strength, and railroads, it becomes clear that the outcome of the war was never really in doubt. In the face of such Northern advantages, the South had to rely on the bravery and determination of its troops and the talent of its generals. But on all other fronts save the central battlegrounds along the Richmond-Washington corridor, the Southern generals proved no more skillful than their Northern opponents. Only Lee and his lieutenants were outstanding, at least until Stonewall Jackson died at Chancellorsville, taking with him some vital spark of energy and military magic.

Lee, the preeminent battlefield commander despite his defeat at Gettysburg, finally met an opponent to match his skill when Grant arrived from Vicksburg to lead the drive across the Potomac to the Confederate capital of Richmond.But in that critical theater of war, where, until Gettysburg, the South held a military advantage, the North’s political vulnerability was most acute. A successful invasion of Maryland and Pennsylvania by Lee’s Army of Northern Virginia would threaten Washington and Baltimore and possibly even Philadelphia. Lee understood this. After he was stopped in western Maryland at the Battle of Antietam, in 1862, Lee mourned, “We would have been in a few days’ march of Philadelphia, and the occupation of that city would have given us peace.” In the spring of 1863, Lee sought a purchase in the North again, writing to the Confederate secretary of war, James Seddon, of his objective: that “next fall there will be a great change in public opinion in the North . . . [and] . . . the friends of peace will become so strong” that the Confederacy would have its independence.

When Lee crossed the Potomac in mid-June 1863, panic began to spread. New military departments were established to defend Pittsburgh (trenches were dug on Mount Washington) and, 160 miles to the east, Chambersburg, which fell to Lee’s troops before any defenses could be begun. The governor of New Jersey wrote to Lincoln that he feared an invasion, and New York sought help to guard its harbor. When General Napoleon J. T. Dana arrived to take charge in Philadelphia, he found only 400 troops; the stores were closed so employees could drill, and the churches open so that citizens might ponder “their duty in this, their darkest hour.” Pennsylvania’s roads were filled with refugees, and the guns of General Richard Ewell were bombarding Harrisburg.

The North’s army, morale battered after successive defeats, was in disarray. On June 28, just three days before the battle, General “Fighting Joe” Hooker was sacked and replaced by General George Meade, even though Lincoln feared Meade was another peace-seeking Democrat like the recently cashiered General George McClellan. The abolitionist press distrusted Meade, as did General Daniel Sickles, one of his corps commanders. (The Meade-Sickles row, which erupted over the way Sickles deployed his troops at Gettysburg, inspired an Army court of inquiry and has become a subsection of Gettysburg studies with a bibliography of its own. A combination of personal animosity, Sickles’s lack of military experience, and the fog of war seems to explain most of the fuss.)

The battle at Gettysburg did not go according to the plan of either commander. Lee had hoped, by taking the strategic offensive, to force the Northern troops to attack him, thus giving him the advantage of the tactical defensive. Attacking had been Hooker’s intention, but Meade was more cautious. He hoped to occupy the strong defensive position of Pipe Creek near Taneytown, Maryland, where he could cover Washington and force Lee to attack him. In the event, both armies blundered into an unplanned encounter about a dozen miles away at Gettysburg, the site of an important crossroads. At first Lee had the numerical advantage, but day by day more Northern reinforcements arrived.

The opposing generals had different priorities. It was enough for Meade simply to hold on; his very presence would abort Lee’s plan to win the strategic political victory by invading the North. But Lee had to defeat the Union army and drive it from the field so that his invasion could proceed. So, even though by the final day he was outnumbered—there were 93,000 Union troops to his 71,000—Lee had to attack. On the first day, he drove the Union troops out of Gettysburg, but they retained a strong defensive position on the ridges south of the town. On the second day, he very nearly turned and broke the Union left flank at Little Round Top, but nearly was not good enough. On the third day, in what now seems a gamble close to an act of desperation, he launched his magnificent infantry uphill over open ground against dug-in defenders with strong artillery support. It is remarkable that this attack, known to history as Pickett’s Charge, came so close to success. But it failed, and the South’s hopes of forcing a political settlement through victory failed with it.

It is one of the great merits of Guelzo’s book that he explains the importance of the tactical defensive. In doing so, he challenges the orthodox view of the distinguished Civil War historian Bruce Catton, who reckoned that the new Minié rifles (available to both sides) had the power and range to dominate the battlefield out to 400 yards and impose crushing losses on the attacker. It would follow from this viewpoint that infantry alone would suffice to hold a position. But if soldiers armed with Minié rifles aimed poorly or were blinded by smoke, infantry could be vulnerable to an attacker who combined artillery bombardment with a bayonet charge—a combination the Southern troops had learned to deploy to great effect. Guelzo points out that the power of the Minié rifle has been greatly overestimated, not least because the clouds of smoke released by a volley of black powder made subsequent attempts to aim difficult. At the height of Pickett’s Charge, Union troops were reduced to aiming at the shoes of their attackers, since nothing else could be seen. Despite claims of two or three aimed shots per minute by men armed with the new Miniés, Guelzo cites reports that soldiers fired much more slowly in battle conditions, as infrequently as once every four minutes.

Moreover, few troops were good shots. Even under perfect conditions, only four members of an Illinois regiment of 180 men could hit a barrel 100 yards away. Only four out of 40 men of the Fifth Connecticut could hit a barn from 100 yards, and only one of those shots was below the height of a man. As the British and French had found in their campaigns in Italy, the Crimea, and India in the 1850s, the decisive power of the infantry rested with the bayonet charge and the volley fire of ranks of infantry at close range. Such was the storm of lead aimed at Pickett’s Charge on the final day that one plank of a fence the Southern attackers had to surmount, measuring 16 feet by 14 inches, was perforated by 836 musket balls, one bullet for every three square inches. Some of those balls probably came from canister, a close-range antipersonnel round fired by cannon, and the thick fire ensured that as General George Pickett’s troops approached the crest of Cemetery Ridge they were rushing into an especially fearsome killing zone.

The real queen of the battlefield was the artillery, and Meade had 372 guns at Gettysburg, while Lee was so short of draft horses that he had crossed the Potomac with only 283 guns. Bursting overhead, shells fired at long range could strike infantry even in cover, while a battery of 18 guns was reckoned sufficient to break up attacking infantry formations. And at short range, the case shot that delivered scores of musket balls was devastating. The impact of artillery has seldom been better explained, nor a battle better described, than in Guelzo’s account of the duel between the guns as Pickett deployed his troops for their fatal charge. To launch a massed infantry assault uphill and across 800 yards of open ground against an enemy superior in artillery was asking more from troops than Lee had any right to expect.

The North had the further advantage of interior lines. Forced out of the town of Gettysburg on the first day, Union troops had been pushed into lines resembling the shape of a fishhook. By the third day, General Henry Slocum’s troops south of Culp’s Hill were but a mile from General Winfield Scott Hancock’s men on Cemetery Ridge. Reinforcements could therefore move quickly from one side of the Northern defensive lines to another. But any reinforcements from Ewell’s Southerners, who had to attack Slocum in order to reach Pickett’s men as they lined up for their famous charge, had five roundabout miles to travel. And so did any dispatch riders trying to coordinate the attacks of the two wings of Lee’s forces, stretched out as they were on exterior lines.

And yet Lee almost won, on at least two occasions. In each case, the fault was his; he failed to concentrate his forces, and he failed to coordinate his attacks. Had Ewell pressed his attack on the evening of the first day, or had General James Longstreet not taken so long (those exterior lines again) to develop his attacks on the North’s left flank on the second day, victory might have gone to the Confederacy. The key feature of this part of the battlefield, Little Round Top, had been left unguarded by Sickles, who had deployed his troops further forward. But the brave Colonel Strong Vincent, who was to die from wounds sustained in the battle, got a brigade of Union troops and some crucial artillery onto Little Round Top with but minutes to spare. Even then, it took heroic bravery by the First Minnesota and the 20th Maine to save the day with desperate bayonet charges against superior numbers. Four out of every five men in the First Minnesota fell in the battle.

Lee avoided a crushing defeat only because of the exhaustion of the Union troops, who had suffered 22,000 casualties over the three days of battle. Lee had lost almost as many, including a third of his 52 generals. Indeed, Guelzo notes that in proportional terms, Lee’s army lost two and a half times as many men as the Allied armies of World War II (British, Canadian, and American) between D-Day and the fall of Paris 11 weeks later.

Lincoln never forgave Meade for failing to turn Lee’s defeat into a rout. “We had them in our grasp,” the president told his son on July 14, when it became clear that Lee had made good his escape. “There is bad faith somewhere,” he told Gideon Welles, his Navy secretary. The war had 21 months more to run, with the slaughters of Spotsylvania, Cold Harbor, and the Crater, the march through Georgia, and the burning of Atlanta still to come. The Union was eventually saved and slavery ended, but at a terrible price whose echoes and political resentments have endured for generations.

History books usually tell us as much about the times in which they were written as about the period they cover, but there is a timeless quality to Gettysburg: The Last Invasion that makes it special. First, it treats the battle as a political as well as a military event, as important to saving Lincoln’s presidency (and dooming Lee’s invasion strategy) as it was to preserving Philadelphia and Pittsburgh from occupation.

Second, the book blends the two sharply contrasting perspectives of battle, from the general trying to arrange ammunition supplies and reinforcements while hearing conflicting reports from his subordinates, to the hapless, hungry, frightened infantryman seeing his fellows fall around him. Perhaps the only thing the two perspectives had in common was that each was mostly blinded by the smoke of battle.

Third, it explains far better than most military histories the importance of ground, of the cover provided by woodlands or the reverse slope of a hill, the difficulty of keeping formation with dead and wounded underfoot and advancing or aiming uphill.

Fourth, Guelzo focuses intently on the importance of time: how much time was required for a report of some new emergency to be drafted and to reach a general, and then, for that general’s orders to reach a unit, and how long it took to assemble and march men and guns to ensure that those orders were fulfilled.

Above all, in an age when most military histories are read by people with no personal experience of the mind-numbing sounds and chaos of battle, Guelzo stresses that raw and terrifying truth Clausewitz tried to convey when he wrote that “the light of reason no longer moves here in the same medium.” All war is hell, but Gettysburg lay in the seventh circle.