This spring, a team of archaeologists and volunteers began painstakingly digging into the history of Fort Mercer, a Revolutionary War fortification on the Delaware River that is now the centerpiece of Red Bank Battlefield Park in National Park, N.J.

During the war, Continental Army soldiers were stationed at the fort to keep the British and their Hessian mercenary allies from resupplying troops in nearby Philadelphia. On Oct. 22, 1777, the army repelled a major assault by Hessian forces. Little-known today, the Battle of Red Bank was brief and ferocious, marking one of the worst defeat the Hessians suffered in the war.

The archaeologists were focused on excavating a trench that had been used to defend the fort during the battle. “My sense was we were going to be looking at the kind of trash that a garrison might throw away,” said Wade Catts, principal archaeologist with South River Heritage Consulting in Newark, Del. Mr. Catts led the dig with Jennifer Janofsky, the director of the park and a historian at Rowan University in Glassboro, N.J.

Instead, at about 2 p.m. on June 26, the last day of field work, the team found a leg bone; they quickly determined that it had belonged to one of the attacking Hessians. It was the first human bone to be found at the site since 1904, when a new fence was built at the battlefield. Over the next few weeks, the group recovered the remains of 14 individuals, which promise to provide scientists with a detailed look at military life and death in that era. “I didn’t really think we were going to get a mass burial,” Mr. Catts said.

On the day of the attack in 1777, the Hessians surely thought the same. The force of 2,300 mercenaries was led by Col. Carl Emil Ulrich von Donop, a courageous leader with a fiery temper, according to letters written by his officers. The fort was defended by only 534 soldiers, including members of the Sixth Virginia Regiment and the New Jersey militia, as well as members of the First and Second Rhode Island Regiments, two of the nation’s first integrated military units. Forty-eight of the American soldiers were Black; the regiments also included Native Americans of the Narragansett people.

Colonel von Donop was confident of victory. Fort Mercer “will be Fort Donop or I shall be dead,” he wrote to Gen. William Howe, commander of the British military forces. When the Hessians arrived at the fort, Colonel von Donop sent an officer to call for the Americans to surrender. “The King of England orders his rebellious subjects to lay down their arms,” the demand stated. “If they stand battle, no quarter whatever will be given.”

The American commander, Col. Christopher Greene, soon replied: The Americans accepted the challenge, and no quarter should be taken on either side. Fighting commenced at 4 p.m. From the river, 13 galleys of the Pennsylvania Navy immediately bombarded the Hessians with cannon fire, and the soldiers inside Fort Mercer opened up with muskets and 14 cannons of their own. Two battalions and one regiment of Hessian soldiers advanced through the barrage. Their assault was slowed by trees that had been cut down; branches had been sharpened and stacked in a line around the fort. The battle lasted just 75 minutes; when it was over, 377 Hessian soldiers — and just 14 Americans — were dead.

The horror of that afternoon was soon apparent to the archaeologists. From an excavation pit 10 feet wide, 30 feet long and four-and-a-half feet deep, they recovered 14 skulls and numerous other human bones. Mr. Catts believes that the soldiers belonged to the Regiment von Mirbach and that they were at the center of the Hessian formations during the assault. The injuries to one soldier, Mr. Catts said, included “a musket ball in the lower part of his back above where his pelvis should be; a lead canister shot in the middle of his back, where he had no more thoracic vertebra; and then a one-and-a-half-inch iron grapeshot that seems to have taken off his left arm.”

Dr. Janofsky noted that the ships on the river were firing chain shot and bar shot at the Hessians, ammunition that is designed to destroy a ship’s rigging. “These guys were being hit by all kinds of things,” Mr. Catts said. “What a horrible place this would have been.”

According to accounts written by surviving Hessian officers, most of the wounded were left on the battlefield: The Hessians had not brought wagons to carry them and the American soldiers, fearing another attack, remained inside the fort. “It is painful for me to lose so many good people, I can’t describe it and I have not recovered from it,” Lt. Col. Ludwig Johann Adolph von Wurmb, who took part in the assault, wrote a few days later. “The tragedy of our poor wounded here in America is not describable without shedding tears, and those left behind with the enemy have no aid.”

That night, a group of American soldiers ventured out to repair some of the defenses. A voice called out from the battlefield: “Whoever you are, take me out of here.” It was Colonel von Donop, who had been shot in the hip.

According to Capt. Thomas Antoine Mauduit du Plessis, the French engineer leading the group, an American soldier shouted out, “Well now, is it agreed that no quarter will be given?” The colonel replied, “I am in your hands. You may take your revenge.” The Americans brought him into the fort and cared for him until he died a week later.

The rest of the Hessian wounded were left where they lay until the next day, when American soldiers were tasked with burying the dead. The ditch in front of the fort may have been an easy place to dispose of the bodies, Dr. Janofsky said. “Are we looking at somebody who was shot, died and is buried?” she said. “Or are we looking at what the burial party did on Oct. 23, 1777, which was essentially pitching bodies into a convenient hole?”

The first human bone recovered, a femur, was found in the excavation pit by Joe Reilly, a self-described history nerd and volunteer, and Wayne Wilson, another volunteer excavator. As soon as it emerged, all digging stopped — the standard procedure when human bones are found. Anna Delaney, the forensic anthropologist for the New Jersey State Police, was called in, and she determined that the femur did not belong to someone who had died recently. Its advanced state of deterioration made that obvious, she said.

In the next weeks, Ms. Delaney helped remove all the human remains from the site and preserved them in her lab, where they will be analyzed and, hopefully, will begin to reveal details of the soldiers’ lives. She and Thomas Crist, a forensic anthropologist at Utica University who has worked on Revolutionary War remains, plan to study the chemical composition of the bones. Certain stable isotopes, and the presence of trace elements, can help determine where a person grew up and what that person’s diet and health were like later in life.

Ms. Delaney and Dr. Crist also hope to recover DNA from the bones and from traces of blood on some of the artifacts. Genetic analysis may allow the researchers to reconstruct the soldiers’ family trees and to learn their identities, Ms. Delaney said: “To be able to give one of these soldiers their name back, to give their family back something, I think that is actually the most exciting part of the whole process.” Once the analyses are complete, the bones will be reburied in a location yet to be determined.

Some of the artifacts recovered from the site tell their own stories. A row of buttons was found, laid out as if they had rested on a coat that was thrown into the trench and that subsequently rotted away. The buttons fit the description of those on the uniforms of the Regiment von Mirbach, Dr. Janofsky said. She suspects that the coat may have been used to transport severed body parts to the trench.

Another intriguing artifact found at the site was a British gold coin, worth about one month’s salary for the average soldier, that Mr. Catts thinks may have belonged to Lt. Col. Ernst Rudolf von Schieck, who commanded the Hessian regiment and died in the fighting.

For Dr. Janofsky, the human remains add poignancy to the story of the battle. Among the dead was a man between 17 and 19 years old, the same age as many of her history students. “Very few of us have seen the violence of the battlefield, and it’s what we’ve been looking at for the past months,” she said. “I feel like we are charged with helping our visitors understand that moment.”