I wasn't aware of any military service until a distant cousin showed me this picture (and I'm not 100% sure it's a military uniform, but that was my first assumption). Any information about the uniform is greatly appreciated. For reference, my 2nd great grandfather lived from 1856 to 1924.
You are correct in assuming it's a military uniform. What your 2nd great-grandfather is wearing is an 1872 pattern U.S. Army dress uniform. The lace stripes on his coat suggest that he was a musician.
The helmet on the table next to him shows that he belongs to either the cavalry, light artillery, or signal corps. I can't tell which of the three due to the picture being in black and white (the color of the trim on his uniform would indicate that). I want to say cavalry, due to the sword he has, and the way it is carried on his belt, but I'm not certain. The helmet plate design was changed in 1881, so this picture would have been taken between 1872 and 1881.
Source: David Cole. Survey of U.S. Army Uniforms, Weapons and Accoutrements. pg 31-32
What nationality was your grandfather ? or do you know which country's military may have served under?