Independence Day is, without doubt, the most important day in the calendar of our national life. It celebrates not the gaining of the nation’s liberty from England, but the date in 1776 when the colonists announced their brash intention to break free.

It was as unlikely to be achieved as it was bold to state, but five years later, after suffering that can hardly be imagined today, the Continental Army, under the command of Gen. George Washington, and aided in no small part by the French, prevailed over the British at Yorktown, Va. It was the last major battle of the war.

Six years later, as the Constitutional Convention concluded its own unlikely work in Philadelphia, a woman asked of Benjamin Franklin, “Well, doctor, what have we got, a republic or a monarchy?” Franklin, as concise as he was wise, responded: “A republic, if you can keep it.”

Keeping it was no sure thing as July arrived in 1863. The nation was in the midst of a catastrophic civil war that began more than two years earlier. In Mississippi, Gen. Ulysses S. Grant was in the 37th day of the siege of Vicksburg, which was the key to controlling the Mississippi River and, with it, the war in the West.

In the East, meanwhile, Confederate forces had marched through Maryland and into Pennsylvania, where, on July 1, they clashed with Union forces in the small town of Gettysburg. It was the start of a frantic and bloody three-day battle that marked the Confederacy’s high-water mark.

They didn’t know it then, but on July 4, 1863, things changed.

Gen. Robert E. Lee was retreating back into Virginia a day after the Battle of Gettysburg climaxed with the slaughter of the foolhardy maneuver known as Pickett’s Charge. It was one of Lee’s few serious mistakes of the war, but it mattered. If it didn’t presage the rebels’ defeat, their army was no longer capable of initiating battles, being reduced to reacting to Union attacks, though often brilliantly.

Also on that day, Vicksburg surrendered. The Mississippi belonged to the Union, and eight months later, Grant was made commander of all Union armies. Other Union generals shied away from conclusive fights. At Gettysburg and the year before, at Antietam, aggressive Union generals might have pursued the rebels and ended the war. In contrast, Grant sought battle with the rebels.

That was 150 years ago today, and thus, this Independence Day carries a special meaning. Not only do we celebrate our independence from Britain, but also the day on which it can be plausibly said that brave men preserved the nation and, equally important, ensured that slavery was in its final throes. It heralded the new birth of freedom that President Abraham Lincoln foresaw that November in his speech at the new cemetery in Gettysburg.

Thousands died in the two battles. At Vicksburg, more than 37,000 were killed or wounded, the vast majority of them Confederates. At Gettysburg, casualties numbered as high as 51,000 – equal to the entire population of Niagara Falls.

At least 620,000 men lost their lives in the line of duty during the war, equal on a percentage basis to about 6 million people today. The carnage can hardly be imagined.

Many more than that have died in the name of the country’s freedom, of course, including 25,000 during the Revolutionary War, 116,516 in World War I and 405,399 in World War II. Some 20,000 died during the War of 1812, whose bicentennial we are observing. Men and women continue to die in Afghanistan – 6,626 total there and in Iraq. And add to that Korea, Vietnam, Mexico and more.

We will have our day off today, most of us. We’ll even be paid for it. But hundreds of thousands of men and women have died for our privilege. Between the burgers and beer and fireworks, we would do well to remember those sacrifices and, especially this year, those that were made in the desperate month of July 1863.