Don Husler was sitting in the shade with the rest of the grimy Yankees, seeking respite from the heat and awaiting the Confederate onslaught, when two quick cannon shots boomed.

“Come on, boys!” someone yelled. The woods came alive. Bugles sounded. The Union soldiers jumped to their feet and grabbed their muskets.

Husler, 61, a retired steelworker who has two artificial knees and a tattoo of the 69th Pennsylvania infantry regiment on his left hand, got up and headed to join his famous unit, and to seal his fate.

“I’m planning on taking a hit,” Husler said. “I always do anyway. I’m too old to fight a whole battle.”

Husler was one of an estimated 11,000 reenactors and 10,000 spectators who swarmed the hallowed area here Sunday to kick off the week’s commemorations of the 150th anniversary of the Battle of Gettysburg.

The ceremonies also mark what is perhaps the high point of the nation’s four-year-long observance of the Sesquicentennial of the 1861-1865 Civil War era.

Thousands of visitors are expected to attend dozens of events throughout the week in and around this historic town marking the July 1-3, 1863, battle that was the turning point of the Civil War.

Sunday’s 12:30 p.m. reenactment, across a huge expanse of farmland outside Gettysburg, sought to recreate the climactic action on the battle’s third day, when Union forces smashed a huge Confederate attack.

It unfolded on a warm, humid afternoon that had reenactors perspiring in heavy woolen uniforms, spectators scorched in the sun and parts of the field obscured by smoke, as if in an old battle painting.

In the nearby woods, which shaded the reenactor camps, the air carried the smell of campfires, cigars and cavalry horses. The parking areas were jammed with cars and pickup trucks, some of which pulled trailers with Civil War-era cannon on board.

Jim Cochrane, 59, an architect from Richmond, had hauled up his brightly polished bronze six-pounder smoothbore cannon behind his Chevy Avalanche. The 900-pound gun tube gleamed in the sun, bearing traces of Brasso polish.

Cochrane said he had just bought the gun for $25,000. It is one of eight that he owns. He and his comrades were portraying a southern artillery unit from King William County, which fought at Gettysburg.

Being here this week “is very, very special,” he said.

Several participants said it was an emotional experience to reenact the disastrous Pickett’s Charge, which the event tried to re-create. The attacking southerners were mowed down as they crossed a mile of open fields.

Shannon Joyce, 40, a funeral director from Bluefield, Va., who was portraying a rebel private in the 24th Virginia infantry, paused as his long column of Confederates snaked down a gravel road and halted to fill wooden canteens.

The men had marched four abreast, their shoes crunching on the gravel, preceded by a fife and drum group.

Clad in a rough uniform of tan, wearing a peaked “mechanic’s cap” and holding a .69-caliber smoothbore musket, Joyce said he had come to Gettysburg “to honor my ancestors that fought in this war, to participate in a once-in-a-lifetime event.”

“I’ve been wanting to do this, Pickett’s Charge, in particular, for a number of years,” he said. “And it’s mixed emotions. . . . It’s very special. It’s very close to all of us. A lot of casualties. . . . I can’t put it into words, what it is.”

Asked if he would stay for the week’s activities, he said: “No sir. I have to go back to work at midnight. I got a seven-hour drive.”

As he spoke, a comrade returned with his refilled canteen. Joyce spat some tobacco juice. Someone shouted an order. The men fell in and the march resumed.

Out on the field, there was a problem with the famous “copse of trees,” which had been the aiming point of Pickett’s attack. The organizers had simulated the road across which the rebels charged and even built a fence like the one on the original road.

But someone had forgotten about the trees. So a gang of Union reenactors retrieved a tree from the woods and hauled it out onto the field. A cheer went up when they raised the tree and stuck it in the ground.

The day’s fight began on schedule, at 12:30 p.m., with an artillery duel. The southern attack started around 1 p.m. By 1:20 p.m., the Confederates were in full retreat.

Earlier, Husler had teared up thinking about “the boys” of the 69th, a heavily Irish unit from Philadelphia, who were engulfed by Pickett’s southerners.

“We’re here for them,” he said. “Basically, that’s the only reason we put our bodies through this. It’s hard work. But it’s in honor of them.”