THE battle of Gettysburg was a human and environmental catastrophe and, for the United States, almost the beginning of the end of its existence. Any historian contemplating this event faces numerous daunting problems - apart from the fact that thousands of books and articles have already been published on the Gettysburg campaign. The ghastly scale of bloodletting has to be explained. The battlefield contest itself involved the highest possible political and national stakes. And although this may be the most documented battle ever, the historian must use a vast supply of unreliable postwar recollections as sources. Such issues never stem the tide of writers drawn by the intellectual challenge (or by the charms of the subject's market). Spread over 15 square miles around a small Pennsylvania town on the first three days of July 1863, involving more than 160,000 soldiers and huge numbers of camp laborers, including between 10,000 and 30,000 slaves forced to serve the invading Southern army, the conflagration caused a degree of slaughter like no other in American history. The Union and Confederate armies officially reported a combined 5,747 dead, 27,229 wounded and 9,515 captured or missing over 72 hours of fighting. Of those wounded, approximately 14 percent died in gruesome impromptu hospitals over the next few months. In his graphic and emotionally affecting "Gettysburg: The Last Invasion," Allen C. Guelzo, a distinguished Lincoln scholar who teaches at Gettysburg College, offers an extraordinarily detailed and realistic account. Guelzo quotes a New York soldier's remembrance, and thereby demonstrates the challenge any military historian faces: "The reality of war is largely obscured by descriptions that tell of movements ... of armies, of the attack and repulse, of victory and defeat. ... All this leaves out of sight the fellows, stretched out with holes through them, or with legs and arms off." Guelzo rises above the carnage and makes clear that in Robert E. Lee's invasion, which threatened Northern cities like Washington and Philadelphia, everything - "the whole war" - was at stake. If Lee's Army of Northern Virginia had decisively defeated (and it nearly did) George G. Meade's Army of the Potomac at Gettysburg, and sent the Northerners into yet another devastating retreat, some version of Confederate victory could have been achieved that summer. With that, we can only imagine how much of American, and even world, history might be different. Guelzo tries to hold these two subjects - three desperate days of combat and the political meaning of the war - in workable tension. Amid the numbing detail about generals' personalities, Guelzo provides some beguiling accounts of the political divisions and rivalries between the officers of the two armies. Both staffs were divided houses, with Lee's corps commanders as well as line officers largely split between Virginians and non-Virginians and between those who had embraced secession and those who had not. The Union Army's officer corps exhibited even more conflict between the "McClellanites," Democrats devoted to the discredited and fired George B. McClellan, who sought a limited war that would never threaten the racial order, and those Republicans of a New England antislavery stripe who really did believe the war must destroy slavery. Guelzo's primary interests, though, are the hour-by-hour waging of this battle; the lower-ranking officers who really fought and died in it; the geography of the green hills, rocky ridges and small springs; and, oddly, those "everlasting and immovable" post-and rail-fences of the freesoil farmers of Pennsylvania. Guelzo maintains that those fences truly stymied Confederate commanders and charging foot soldiers, and were instrumental in determining the outcome. Moreover, Guelzo reveals the sheer confusion of Civil War battles; commanders desperately lacked information about the locations and intentions of their enemies, even sometimes of their own divisions. He quotes Meade insisting that battles are "often decided by accidents." The tide of conflict, especially on the second day, was often a "directionless murk," Guelzo writes. In what becomes a central theme of his narrative, the battle is suddenly punctuated time and again by "miracle moments," when Union troops appear in the nick of time to narrowly save a position at Little Round Top, on Cemetery Ridge or Culp's Hill. Guelzo is no Lee lover; indeed, he leaves Lee largely out of the story, except for arguing that it was his failure to coordinate his generals that caused his defeat. Lee "lost a battle he should have won," Guelzo writes, even calling the Marble Man "shameless" in blaming his own troops for the loss. Meade fares no better; he was ineffective, almost "entirely reactive" and far too cautious. Guelzo denies that the Civil War was either "total" or the first "modern" war. This was 19th-century warfare, he asserts, with the highly inaccurate, if destructive, rifled musket the new weapon of choice. As for artillery, it does seem a moot point to the decapitated victims of solid shot or to those obliterated beyond identification by canister shells to say that Civil War technology was "neither sufficiently accurate nor sufficiently destructive to wreak the kind of obliteration which Krupp guns and aerial bombings would visit on European cities and towns in the 20th century." The blood spread all over the Gettysburg landscape was unparalleled by the standards of the time, whether the product of "modern" technology or not. While admitting the serious limitations of soldiers' postwar reminiscences, Guelzo, with exhaustive research, nevertheless relies on the mass of retellings embedded in regimental histories, memoirs and letters collected from survivors. This helps him make sense of combat, especially its horrific sounds and the smells of its aftermath. To the familiar question, how could those men stand and fight - in the "elbow-to-elbow line" - under such fear and gunfire, he provides a simple answer. Regiments were held together by trust as much as by leadership: "The Civil War soldier needed to know one thing above all others - that the men on either side of him would not run." This book's considerable achievements, though, are marred by Guelzo's literary style, as well as by his apparently irresistible romantic urge to add one more panegyric to the epic of Gettysburg. His claim that military historians have to struggle for respect among the "Civil War's cultured despisers," that a book like his violates "fashion" because it is not about the "agency" of black emancipation, seems unnecessary at best. (He and I have differed on this point before.) And the gems of detail from Guelzo's keen researcher's eye (he tells, for example, of a Roman Catholic priest standing on a rock offering absolution to the silent soldiers as they file into battle, bands playing to raise morale during combat, a Confederate general desperately breaking his sword on the ground as he surrenders) often pale next to the array of recorded descriptions of how soldiers were shot, where bullets penetrated their bodies or those of their horses - "through the left lung," "entered the left side of the stomach, perforating his sword belt and lodging in the spine" and the like. Such passages seem awkwardly clinical when overused, even if garnered from a soldier's remembrance. THERE are other problems with Guelzo's language as well. What does it mean for a full division of infantry to get a "collective bloody nose," or a brigade to have its flank "slapped." After portraying such "sheer carnage," why term this deadly affair a "gigantic boxing tournament gone wildly into three-digit extra rounds?" The historian John Keegan calls this kind of rhetoric the "Zap-Blatt-Banzai-Gott im Himmel-Bayonet in the Guts" style of military history. Guelzo appropriately ends his book with Abraham Lincoln's journey to Gettysburg to deliver his famous address on Nov. 19, 1863. With some effective if overwrought prose, he interprets the speech as Lincoln's defense of liberal democracy and self-government - the cause somehow worth all the sacrifice. By invoking the label "the tall man," rather than Lincoln's name, 10 times in his final few pages Guelzo may have been modeling the great historian Bruce Catton, who used the phrase deftly and beautifully, but only once, in writing so lyrically of the same event in his 1952 Civil War volume "Glory Road." If so, Guelzo should have channeled Catton even more. David W. Blight's latest book is "American Oracle: The Civil War in the Civil Rights Era." He is writing a new biography of Frederick Douglass.