WASHINGTON—For nearly 150 years, a story has circulated about a hidden Civil War message engraved inside Abraham Lincoln’s pocket watch. On Tuesday, museum curators confirmed it was true.

A watchmaker used tiny tools to carefully pry open the antique watch at the National Museum of American History, and a descendant of the engraver read aloud the message from a metal plate underneath the watch face.

“April 13 - 1861,” the first line reads, “Fort Sumpter (sic) was attacked by the rebels on the above date. J Dillon.” The second part repeats same date, states the location as Washington and says, “Thank God we have a government.”

The words were etched in tiny cursive handwriting and filled the the space between tiny screws and gears that jutted through the metal plate. A magnifying glass was required to read them.

Jonathan Dillon, then a watchmaker on Pennsylvania Avenue, had Lincoln’s watch in his hands when he heard the first shots of the Civil War had been fired in South Carolina. The Irish immigrant later recalled being the only Union sympathizer working at the shop in a divided Washington.

Dillon’s story was passed down among his family and friends, eventually reaching a New York Times reporter. In a 1906 article in the paper, an 84-year-old Dillon said no one, including Lincoln, ever saw the inscription as far as he knew.

Dillon had a fuzzy recollection of what he had engraved. He told the newspaper he had written: “The first gun is fired. Slavery is dead. Thank God we have a president who at least will try.”

For years the story went unconfirmed.

The watchmaker’s great-great grandson, Doug Stiles, first heard the tale of the engraving from his great uncle decades ago. He said the story had reached extended family as far away as Ireland.

A few months ago, he used Google to find the New York Times story, and last month he passed the information along to Smithsonian curators, who knew nothing about the engraving.

On Tuesday, watchmaker George Thomas, who volunteers at the museum, spent several minutes carefully opening the watch as an audience of reporters and museum workers watched on a video monitor.

“The moment of truth has come. Is there or is there not an inscription?” Thomas said, teasing the audience, which gasped when he confirmed it was there. He called Stiles up to read his ancestor’s words, drawing smiles and a few sighs of relief.

“Like Pearl Harbor or 9/11, this was the reaction he had (to the Civil War,)” Stiles said of the inscription.

Later, Stiles said he felt closer to the 16th president.

“My gosh, that was Lincoln’s watch,” he said, “and my ancestor put graffiti on it!”

Lincoln’s family kept the watch until it was donated to the museum in 1958. It was Lincoln’s everyday pocket watch, one of the president’s only valuable possessions he brought with him to the White House from Springfield, Ill., said Harry Rubenstein, curator of the museum’s politics and reform division.

“I think it just captures a bit of history that can transform you to another time and place,” he said. “It captures the excitement, the hope of a watchmaker in Washington.”

The watch will go back on display at the museum by Wednesday as part of the exhibit, “Abraham Lincoln: An Extraordinary Life.” It will have a new label to tell Dillon’s story and a photo of the inscription.