During the American Civil War, he was a Union Army officer and many years later, Post received the Medal of Honor on March 8, 1893, for his actions at the Battle of Nashville.

[1] At the beginning of the American Civil War, he entered the Union Army and served with the 59th Illinois Volunteer Infantry Regiment as a 2nd lieutenant.

On December 16, 1864, in a charge on Overton Hill, during the Battle of Nashville, a grapeshot crushed through his hip, making what was for some days thought to be a mortal wound.

[1][2] On March 11, 1865, President Abraham Lincoln nominated, and the U.S. Senate confirmed, Post for appointment to the brevet grade of brigadier general of volunteers, to rank from December 16, 1864.

[3] After the surrender at Appomattox Court House, he was appointed to the command of the western district of Texas, where there was then a concentration of troops on the Mexican border.