Congratulations! That's a good find.

The story that you tell is unusual but not unheard of. Every Confederate state furnished at least one regiment of soldiers for the Union, and Arkansas was no exception. According to the 1860 U.S. census, Arkansas had the third-smallest white population of the Confederate states, yet it still furnished 48 regiments for the Confederacy and 11 for the Union, according to the 1866 records of the state's adjutant general.

Many of those regiments were of black soldiers: More than 5,000 black soldiers from Arkansas fought in the war, and many thousands more served in noncombatant roles.

Among white soldiers, there was initial reluctance to form entire Arkansas units: There was a thought that once armed and equipped, they might prove disloyal in part or in whole, thus jeopardizing the trans-Mississippi. That fear was never realized.

Until the first Arkansas units were formed in early 1862, many loyalists from the state chose to enlist in regiments from Illinois or Missouri. Illinois was a short upriver trip away, while Missouri was closer for those farther inland.

The fighting in this section of the country was particularly brutal and chaotic until Union successes pushed the fighting farther south from 1863 onward. In Missouri and Arkansas (as in many other border and near-border areas) it was often neighbor against neighbor, with one home favoring the Union and another favoring secession.

One of the mistakes folks make when studying the Civil War is to think the lines are drawn between slave/free, Confederate/Union, or that everyone on one side of a line was willing to fight for that side's particular cause. The Rivers Ran Backward: The Civil War and the Remaking of the American Middle Border is a really good (though technical) look at why that's a lie and the detailed, home-level view of the war, including the situation your grandmother's great-uncle might have found himself in.