The phrase "the last full measure of devotion" is a reference to the line from the poem "The Charge of the Light Brigade" by Alfred, Lord Tennyson, which reads: "Into the valley of Death rode the six hundred." This line describes the bravery of the soldiers who fought in the Battle of Balaclava during the Crimean War. Lincoln used this phrase to compare the sacrifice of the soldiers who fought in the Civil War to the sacrifice of the soldiers who fought in the Battle of Balaclava.
Lincoln's use of the phrase "the last full measure of devotion" is a powerful way to describe the sacrifice made by the soldiers who fought in the Civil War. It is a reminder that these soldiers were willing to give up their own lives in order to protect the Union, and that their sacrifice should never be forgotten.