It didn’t have to look like this. (Associated Press)

History, especially military history, often serves us best as a record of how things might have been different.

Visit any major battlefield of the Civil War and you instantly find yourself deep in a thicket of if’s. What if Pickett’s Charge had worked? What if the Union troops had pursued the Confederate forces after Antietam? What if Stonewall Jackson had survived? Everything hinged on the slightest variations of terrain and temperament. The battles whose names have the ring of foregone conclusions — Antietam, Gettysburg, Waterloo — are often long concatenations of coincidences. The people who took advantage of suddenly presented opportunities prevailed; those who missed them, failed.

Gettysburg, as George Will points out, is a place where things might have gone very differently indeed.

July 1 marks the 150th anniversary of the first day of this decisive battle.

One the evening of Tuesday, June 30, troops massed blindly around the little town of Gettysburg in Pennsylvania. They did not realize it was the turning point of the war in the east. All they knew was that the war had been going on for three years and that recently it had been going quite badly for the Union. The Union army in the east was coming off a string of defeats — Fredericksburg, Chancellorsville. Confederate Gen. Robert E. Lee decided to bring the war north.

Lee’s army (about 70,000 strong) faced Union forces (about 100,000 strong) under the command of newly appointed Gen. George Meade. Abraham Lincoln, as commander in chief, needed Meade to work. He was coming off a string of failures. George B. McClellan, “Little Napoleon,” who would later run against Lincoln for president, had done a bang-up job of drilling the troops, but seemed loath to actually use them. Lincoln at one point sent him a note that allegedly read, “Dear General — if you are not using the army, I would like to borrow it for a while.” McClellan had failed to pursue Lee’s army after the bloody battle of Antietam in September 1862, closing the possibility of a decisive blow against the Confederates the year before.

Meade was taking over for Joseph Hooker, who in turn had taken over for Ambrose Burnside (he of the distinctive facial hair), although much of the recent fighting at Second Manassas had been under the supervision of John Pope, a somewhat unsatisfactory leader who liked to remark that his headquarters were in the saddle, prompting others to retort that he kept his headquarters where his hindquarters should be.

Compared to these generals, Meade was a marked improvement. Then again, a defective lemon on horseback might also have been a marked improvement over someone like Hooker.

Lee was headed north hoping to win a victory on Union soil. The Confederacy could not hope to win on numbers or overall manufacturing prowess, but a dramatic victory could  draw foreign intervention or stoke the fires of Northern anti-war sentiment as the war came closer to home.

Lee was a formidable commander. He had been second in his class at West Point and had gone on to be superintendent of the military academy, and was described glowingly by those who had seen him in the Mexican-American War as the “gallant, indefatigable Captain Lee.” Among his other memorable pre-war assignments had been subduing John Brown’s raid on Harper’s Ferry. When the war broke out, Winfield Scott offered him command of the entire Union army.  After a long and agonizing night, he turned it down. Virginia seemed likely to secede, and he did not wish to draw his sword against his native state. If only he had, things might have ended much sooner.

The Lee who marched into Pennsylvania had just lost one of his most trusted lieutenants, Thomas J. “Stonewall” Jackson, to friendly fire at the battle of Chancellorsville. Jackson was a dour Calvinist whose tactical acumen had been crucial in executing Lee’s more daring maneuvers. (Fun Jackson Fact: There is a window at the National Cathedral labeled “The Apotheosis of Stonewall Jackson” for anyone who wants to see a Confederate general being taken up into The Light in a suit of stainless armor. It was donated by the Daughters of the Confederacy.) At first it looked as though Jackson might pull through but have his left arm amputated. Lee responded to the news by saying, “He has lost his left arm; but I have lost my right arm.” Jackson’s absence supplies one of the more potent if’s of the Gettysburg campaign.

The corps commanders who succeeded him were not incompetent, but they were no Jacksons either — James Longstreet, A. P. Hill, and Richard S. “Baldy” Ewell, missing a leg — and tended to be more prone to question and hesitation. The army was in good fighting shape, although short on supplies.

Lee’s cavalry, under the command of the flamboyant and dashing Maj. Gen. J. E. B. Stuart, was not in touch with the army. Stuart was an excellent cavalry commander but prone to stunts. He had decided it would be a good idea to ride around the Union army, since the last time he had done this he had wreaked havoc, caused thousands of dollars of damage and earned numerous headlines. This time went less well. He suffered his first setback in battle at Brandy Station and got out of touch with the bulk of the army, leaving Lee groping blindly up the countryside, unaware that Meade had taken command of the Union army from Hooker or that the army had crossed back over the Potomac and was shadowing his movements.

Gettysburg was, and remains, a slumberous town, although now there are more Pickett’s Charge-themed buffet restaurants than I assume there were at the time.

But as Lt. Col. Joseph B. Mitchell noted in his “Decisive Battles of the Civil War,” “The selection of the site for the greatest battle ever fought on the American continent was an accident.”

The day before the battle, troops skirmished. Confederate troops who had wandered into town seeking shoes ran into Union cavalry under the command of Gen. John R. Buford. Buford was swift to realize that the ground he occupied — the heights of Cemetery Ridge — was the best ground. If a battle were to happen near Gettysburg, which seemed increasingly likely, holding the heights might be the difference between victory and defeat. Gettysburg is located at the intersection of a number of major roads and, in retrospect, it made perfect sense as a location for a battle. All roads lead to Rome, maybe, but if you’re in the Gettysburg vicinity, all roads lead to Gettysburg. Glance at a map, and the selection of place seems almost inevitable. Almost.

Buford told his troops to dig in. It was the first of many pivotal moments in three history-making days. In the early morning darkness of July 1, they waited for the attack from one side and reinforcements from the other.

This was just one moment when things could have turned out very differently. The next days would be full of them. Even 150 years later, these July days stand out as a time when the course of history changed through luck, location and the courageous actions of a multitude of individuals. It only seems obvious in retrospect.

Feel free to join the small battalion of people following me on Twitter, where I’ll be live-tweeting my adventures in the field and off it, on the 150th anniversary of this climactic battle.

Alexandra Petri writes the ComPost blog, offering a lighter take on the news and opinions of the day.