From: (Secession On Trial, Cynthia Nicoletti, Cambridge University Press, 2017)
"Animated by a widespread Southern secession movement to preserve the institution of slavery, the war demonstrated Northerners' commitment to maintain the Union intact. Nevertheless, it rmained necessary for Northerners and Southerners to determine the war's legal significance in the wake of Union military victory. " p. (18)
"In 1865, Americans were unsure whether Union victory in the field would stand as the definite pronouncement of secession's illegality or whether Davis's case would provide an alternate answer to that question." p. (18)
"The war had prevented the disintegration of the Union, but many feared that the 'verdict' of the war would not survive a legal challenge in a regularly constituted court of law." p.(19)
Thus the Jeff Davis case would be chosen to prove the treason of secession, but once the lawyers made the preparations for that trial they feared it would be found in the South's favor. So, rather then taking that chance, the were willing to let Jeff Davis go free.
These are just a few quotes. If you want more, let me know.
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