A purloined letter returned

The neatly handwritten two-page letter by Joseph Warren describes a pivotal American victory at Fort Ticonderoga, which provided momentum to help colonists drive British troops out of Boston months later, an event commemorated as Evacuation Day.

“It is an important part of our history and was probably the last official letter he wrote before he was killed in the Battle of Bunker Hill,’’ said William F. Galvin, secretary of the Commonwealth. “It ties together a lot of signif icant historical events.’’

Warren wrote the letter in Watertown on May 25, 1775, three weeks before he was killed at the age of 34.

The letter was one of many stolen from the state archives, which at the time were located in the basement of the State House. Documents were filed in books similar to photo albums, and security was lax. Most of the thefts were not discovered until after the archives moved to a high-tech granite building at Columbia Point in Dorchester in the 1980s and a comprehensive inventory was taken.

The letter ended up in the collection of a well-known gatherer of Americana, James S. Copley, a newspaper publisher from San Diego, who died in 1973. Last fall, part of Copley’s showcase, including Warren’s letter, was put up for sale by Sotheby’s.

Archivists here noticed the sale on the Internet. The Commonwealth negotiated with Copley’s estate and paid $8,000 for the document. Authorities were certain that Copley did not have a hand in the theft six decades ago, but rather came across the document after it had probably changed hands several times.

The letter, returned November but announced just yesterday, was in good condition but part of the top that contained the archive’s stamp had been cut off.

Warren was born in Roxbury, attended Roxbury Latin School, and graduated from Harvard College. He married, had four children, and worked as a physician in Boston. His wife died eight years after they married.

Warren gravitated toward politics and was one of the first members of the Committees of Correspondence, organized “to keep the unrest alive,’’ said Michael Comeau, assistant state archivist.

After the Boston Massacre, Warren gave rousing addresses to stir up anti-British sentiment. He was appointed president of the Massachusetts Provincial Congress, the highest position in the Revolutionary government, and participated in the battles of Lexington and Concord.

“What makes this letter so historically significant is that it hits the trifecta of historical documents,’’ Comeau said. “You have a very important person describing very significant events during extraordinary times.’’

Warren’s letter has references to Colonel Benedict Arnold, Ethan Allen, and General Henry Knox. Arnold and Allen’s soldiers joined to capture Fort Ticonderoga.

In a postscript, Warren asks that the good news be shared with Knox.

Ten months later, Knox’s troops, using cannons and other armament taken from the British at Fort Ticonderoga, forced them out of Boston.

Warren writes that Arnold, because he didn’t have “the sole honor’’ of taking Fort Ticonderoga, would probably attempt to take another fort on his own.

“It was a foreshadowing of an ego that eventually led him to become a traitor,’’ Comeau said.

Brian R. Ballou can be reached at bballou@globe.com.