Arkansas had strong regional divisions, and so also was divided over whether to secede. It had flatlands on the Mississippi river in the southeast, which had many big farms and cotton plantations, where slavery was common: this region was pro-Secession. But it also had hilly areas in the northwest, called the Ozarks, where slavery was not as common and not as profitable. The Ozarks were much more pro-Union.

It was one of several southern states that was divided this way. The western part of Virginia and the eastern part of Tennessee were also more pro-Union: the western part of Virginia was unified enough to form a separate government, and eventually the state of West Virginia. Eastern Tennessee was never able to go that far ( though Lincoln's vice-president Andrew Johnson was from there, who would later become president on Lincoln's death). Arkansas's Ozark region was not unified enough to actually form a separate state, but like east Tennessee, western Virginia, northern Georgia and northern Alabama, it would contribute significant numbers of troops to the Union army.

Following the Emancipation Proclamation on Jan. 1, in May of 1863 the Adjutant General of the US Army authorized the creation of the Bureau of Colored Troops. That Bureau then enlisted more than 180,000 Black soldiers, 5,526 of them in Arkansas. Exactly how many of these were free Blacks and how many were liberated slaves is hard to say. But free Blacks numbered around 700 in the state before 1859, and all of them were actually ordered to leave the state in that year. A bare 140 were able to hide, and were resident after 1860. The Union was able to take extensive control over much of the state during 1863, and anywhere the Union Army took control, the enslaved population was liberated. So, pretty much all of the 5,526 soldiers would have been liberated slaves.

Report of the adjutant general of Arkansas, for the period of the late rebellion, and to November 1, 1866. This is from the very large and very important collection, Official Records of the War of the Rebellion, but the link is to an ASCII-generated copy that has many errors , so I am sorry to say the orthography is even worse than, well, the standard English orthography!

[Black Union Troops in the State of Arkansas](https://encyclopediaofarkansas.net/entries/black-union-troops-5135/#:~:text=Regiments%20of%20Black%20soldiers%20were,States%20Colored%20Troops%20(USCT). Encyclopedia of Arkansas.