Answer: the Union cause.
Step-by-step explanation:
Although never actually mailed, Sullivan Ballou´s letter, written to his wife, Sarah, shortly before he was mortally injured, made him a well-known character of the Civil War. Sarah did receive other letters from her husband, but this one was only found among Sullivan's belongings after Governor William Sprague of Rhode Island went to Virginia to recover the remains of his state's citizens fallen in battle.