FORT WORTH — Millions of Texans have read the “Victory or Death” letter written at the Alamo more than 170 years ago. But only a small number of them have ever laid eyes on the original — a brief plea for reinforcements written by Lt. Col. William Barret Travis on Feb. 24, 1836, as he and his outnumbered men faced the Mexican Army.

Whether it ever returns to the Alamo is now a hotly debated issue.

The letter has become one of the most revered documents in Texas history, and one of its phrases — “Victory or Death,” which Colonel Travis underlined three times — has endured as an unofficial Texas slogan, turning up on flags and, occasionally, in the speeches of politicians, including one that Gov. Rick Perry gave last year as he campaigned for president....