Yep a General Service Button. Probably Civil War period. We'll need to see the back to get an approximate date.

I learn something new on Tnet all the time. I didn’t know that about the shield.Fyrffytr1 is correct... this US Army General-Service "Enlisted-Men's ranks" uniform button is from the Indian Wars era. Specifically, this button is the M-1874 version and it was issued from that year until it was replaced in 1902 by the "Great Seal of the United States" version of eagle-button.
A key ID-clue for this version is the highly raised shield, which Fyrffytr1 already mentioned. on the civil war era version of this button, the shield is not nearly as high-raised... but note, there are one or two exceptions to that rule.
Fyrffytr1 is correct... this US Army General-Service "Enlisted-Men's ranks" uniform button is from the Indian Wars era. Specifically, this button is the M-1874 version and it was issued from that year until it was replaced in 1902 by the "Great Seal of the United States" version of eagle-button.
A key ID-clue for this version is the highly raised shield, which Fyrffytr1 already mentioned. on the civil war era version of this button, the shield is not nearly as high-raised... but note, there are one or two exceptions to that rule.