Kentucky was a border state during the American Civil War and one of the northern-most “Confederate Heartland” states as well. The Confederate Heartland (i.e., Western Theater) is noted by modern historians as that portion of “the vast region south of the Ohio River and between the Appalchian Mountains on the east and the Mississippi River on the west (McMurry).”
The most northern portion of the Confederate Heartland would most significantly be the states of Kentucky and Tennessee. Whomever controlled that northern portion would be in a great position to also control the entire Confederate heartland.
By early 1862 the Union held firm control over Kentucky and Tennessee. Nashville capitualted in Feb 1862 without a shot being fired. What did this result in? By gaining control of the northern heartland Kentucky was not likely to ever secede and the capture and occupation of Nashville – from early 1862 onward – meant that the Confederate states would be deprived of the:
“South’s great horse countrymost of the Volunteer State’s raw materials (notably iron and copper), its significant industrial capacity, its railroads. and its great agricultural production (McMurry, May 2012 issue (Vol 14 #1) of North and South Magazine, ”From the West . . . Where the War Was Decided.”
Also see: Sister States, Enemy States : The Civil War in Kentucky and Tennessee. Kent Dollar, ed. Univ of Kentucky Press, 2011.