That specific button, which is made of zinc, was issued on US Army uniform pants in the World War One era. Yours, whose front says "U.S. [star] Army [star]" is the size used for the pants' waist-closure. A slightly smaller size, which has "[star] U.S.A. [star]" on it, was used as the pants' fly-closure buttons. The photo below shows both versions, on the uniform pants, along with the pants-manufacturer's contract-dated label, March 15, 1918.
Although in the past they've been incorrectly identified as being from the civil war era, they definitely are from the World War One era, and in use "here and there" for a decade or two afterward.
I notice that the photo at the link helpfully posted by my good friend and colleague Nhbenz shows a 1930s blue denim "Chore Jacket" with a mismatched group of US-marked zinc 4-hole buttons and wreath-marked brass 4-hole buttons. In my opinion, it is unclear whether the zinc ones or the brass ones were the replacements when some of that jacket's "original" buttons were lost.