A recent edit to Has any of the US federal armed forces acted violently against civil movements / protesters? invalidates several answers with multiple upvotes. The original question had no date threshold. The choice of WWII as a threshold by an editor seems fairly arbitrary. My understanding from this question Question editing that invalidates answers is that edits that invalidate good answers are to be avoided. Have I misunderstood? I don't want to get into an edit war with anyone.
The edit has been reversed, so this is no longer an issue.