Well David, once again, just because Hollywood included it in a movie, doesn't necessarily make it true. With a little Googling here is one description of how the letter came about:

"In the fall of 1864, Massachusetts Governor John A. Andrew wrote to President Lincoln asking him to express condolences to Mrs. Lydia Bixby, a widow who was believed to have lost five sons during the Civil War. Lincoln's letter to her was printed by the Boston Evening Transcript. Later it was revealed that only two of Mrs. Bixby's five sons died in battle (Charles and Oliver). One deserted the army, one was honorably discharged, and another deserted or died a prisoner of war.

"The authorship of the letter has been debated by scholars, some of whom believe it was written instead by John Hay, one of Lincoln's White House secretaries. The original letter was destroyed by Mrs. Bixby, who was a Confederate sympathizer and disliked President Lincoln. Copies of an early forgery have been circulating for many years, causing many people to believe they have the original letter."

I can't find my notes right now, but several years ago I called the Rare Manuscripts division over at the Library of Congress and asked the question: Did A. Lincoln write to Mrs. Bixby? The man (whose name I can't find right now) answered with a laugh. "No, he did not!"

Alice Gayley