I found a similar one a few weeks ago too. Looks to be WW1, great seal button and general service like nuggetdog said.
Any writing on the backmark? Mine is a Waterbury. What it has will help provide a date. Older ones will have Scovill or Waterbury and Scovill with a combination of the word “button”, “co” and/or “conn”
Your button is a 1902 Great Seal button that is still in use today. Most of those that are dug, are either WW1 or WW11 era. There are little ways to tell the difference as to which time era or war it may have been involved in. You can try Google, or Albert's button book.
I just looked at my Viet Nam era uniform and the design on the buttons is the same. However, they are still shiny brass because they're still on the uniform!