WASHINGTON (Reuters) - A gold watch owned by Abraham Lincoln bears a message marking the start of the U.S. Civil War, but the president never knew of the "secret" inscription uncovered on Tuesday at the National Museum of American History.

The engraving, by watchmaker Jonathan Dillon, is dated April 13, 1861, and reads in part: "Fort Sumpter was attacked by the rebels" and "thank God we have a government."

The museum said it agreed to open the watch to find out if the message really was there after it was contacted by the watchmaker's great-great-grandson, Doug Stiles of Waukegan, Illinois.

The American Civil War began when Confederate troops opened fire on Fort Sumter in Charleston, South Carolina, on April 12, 1861.

Forty-five years later, Dillon the watchmaker told The New York Times that he was repairing Lincoln's watch when he heard that the first shots of the Civil War had been fired.

Dillon said he unscrewed the dial of the watch and used a sharp instrument to mark the historic day on the president's watch. He told the newspaper that, as far as he knew, no one had ever seen the inscription.

"Lincoln never knew of the message he carried in his pocket," Brent Glass, director of the National Museum of American History said in a statement. "It's a personal side of history about an ordinary watchman being inspired to record something for posterity."

Lincoln was elected the 16th president of the United States in November 1860. In the leadup to the Civil War, South Carolina and six other states seceded from the Union before Lincoln's inauguration in March 1861.

(Reporting by JoAnne Allen; Editing by Patricia Zengerle)