There is no reference on the internet to any poem or short story by
Robbins entitled "Soldier's Reprieve".
However, in addition to McGonagall's peom
there is a touching short story and another poem with the same title
and theme: the story of a girl who pleads with Abraham Lincoln to
spare the life of her brother who is about to be shot for sleeping at
his post whilst on sentry duty.
The author of the short story is unknown, although it is often
credited to the New York Observer in which it has been printed. You
can read it on several websites, including these:
bygosh.com Featured Short Story
The other version of the poem is credited to Rose Hartwick Thorpe
writing in the Detroit Free Press, and tells a slightly different
story from McGonagall's version. It was published in John Hill
Aughey's "Tupelo" in 1905, and can be found starting at the bottom of
page 459 in this massive HTML reproduction of the book:
John Hill Aughey, "Tupelo"
An illustration from "Tupelo" of the girl with Lincoln is here:
I trust that you find these references useful. Please request
clarification if this answer does not yet meet your needs.
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