After the Civil War, a former lieutenant of the Union Army, artist James Hope, took up painting scenes from his notebook of sketches of the war that pitted brother against brother.

One of the most famous results was “The Aftermath,” which depicted the gory trench known as Bloody Lane that was said to have been piled six deep with bodies after the battle of Antietam on September 17, 1862. The battle was the bloodiest in America’s history, with about 24,000 men dying there in Maryland or soon after from the injuries they sustained on the field. It also was a major victory for the Union Army, emboldening President Lincoln to abolish slavery in the South.

Arguably the most important painting by Hope, “The Aftermath” was thought to have been washed away in the great Elmira flood of 1937. The house in Watkins Glen, N.Y., where Hope retired was buried by tons of mud and silt.

Years later, a portion of “The Aftermath” and other of his paintings were dug up from the estate and hung in the eaves of a local church. They were later bought by the National Park Services and restored, and are now exhibited at the Antietam National Battlefield museum. The other portion of “The Aftermath” was considered lost forever.

Recently, however, the bottom-left section of the painting, a 52- by 70-inch piece that bears the most detailed image of the bloody trench and Hope’s signature, has surfaced from the collection of a private art collector in the city. The missing section, which is water-damaged but still discernible, was found in 1998 or so behind a cupboard in a home near where Hope died, a lawyer who represents the collector, Michael Collesano, said. The owner of the cupboard had no idea what it was, he said.

Eventually, after passing through several art dealers, the fragment end ed up in the hands of the collector, who has held onto it over the years because the National Parks Service didn’t have the money to buy and restore it. However, the archivists and historians at the National Park Service did verify the fragment as belonging to the original.

“The two pieces fit together perfectly,” an art dealer who also represents the collector, Dennis Insalaco, said.

He and his client, who Mr. Insalaco said preferred not to be named, are attempting to get private investors to put up the money to reunite this section of “The Aftermath” with the remainder at Antietam. They are seeking $400,000 to buy the painting and another $100,000 to restore it, Mr. Collesano said.

“It will probably take a Civil War buff to understand the importance of the donation,” Mr. Insalaco said. “This is a remnant that will give us a really clear idea of what’s considered Hope’s best and most important painting.”

The museum currently displays an artificial image of the painting that is based on a photograph taken before the flood. Using high-tech graphical tools, they have recreated what the painting may have looked like before it was damaged.

Nothing can compare to actually uniting the two pieces of the original, Mr. Insalaco said.

The National Parks Service’s superintendent of the Antietam Battlefield and Museum, John Howard, said the painting “showed the enormity of the fighting here.”

He said a former deputy secretary of defense, Paul Wolfowitz, who is now president of the World Bank, found the painting so meaningful that he kept a copy of it in his office.

“It is a beautiful work of art that reminds people of the horror of war,” Mr. Howard said.

As a scout and mapmaker, Hope often was on the hills surrounding battlefields and was said to make quick sketches in pencil whenever the opportunity arose. He created his large murals of the battles by combining many images from his notebooks. The effect was to create a painting that told a story over a period of hours, rather than capturing a single moment, Mr. Howard said.

“These paintings were done by somebody who was there,” he said. “We find at the museum that this is the most important part” for people learning about the Civil War.