The time of the Spanish American War saw the final pardons of all ex-Confederates, which then allowed several major Confederate officers to be commissioned into the U.S. Armed Forces. It was a short war and albeit small too. All the ex-Confederate states are listed with providing units to serve. I don't believe it was very popular to serve in the Federal military services in the South up until that time. State militias were still very popular.

A while back I had already looked up the census records of Army post in 1870 and 1880 and found only a handful of Southerners in the U.S. Army (out West). 90% or so were foriegners.

http://www.spanamwar.com/units.htm

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David Upton