It is the 1854-to-1875 version of a US army "general service" button for Enlisted-men (non-Officers). Because the army was very small before the civil war and soon after it ended, the statistical odds favor your button being made during the war. But as G.A.P.metal indicated, a maker's-mark ("backmark") on your button's back might enable us to give a narrower time range. Please post a well-focused closeup photo of the back, and if it has a backmark tell us anything you can read there.
Okay, when I get home I'll get an image of the reverse of the button. Thank you all very much, I've never been on a forum before and it's like having a panel of experts at your fingertips, who'da thunk?
Thanks for the photo of the back of your civil war era US army "eagle button." The blank back with a depressed "bowl" around where the loop went is a characteristic of ones which were mass-produced as cheaply as possible during the height of the civil war. In other words, your yankee Enlisted-man's button was made during the war, not before or after.
Thanks Cannonball,
Hopefully I'll find some confederate buttons as well. I have two more that I found but I believe they are ww1&2 naval buttons. If this ground would thaw I could get back to it!