‘O Israel, you have sinned from the days of Gibeah.
There they took a stand.
Did not the battle against the children of iniquity
Overtake them in Gibeah?'
What had happened at Gibeah (Judges 19-21) was written deep into the heart of Israel. There Israel, in the person of the tribe of Benjamin, had sinned deeply in a perverted sexual way. And YHWH now tells Israel that they have not changed one iota. They have continued to sin in the same way since the days of Gibeah. They are no better than the people whom YHWH destroyed there.
In Gibeah Benjamin had taken its stand, and was in fact routed, and subsequently almost destroyed. The children of iniquity were truly overtaken in Gibeah. But what YHWH is saying here is that that was not the end of the battle against perverted sexual sin, for there were still children of iniquity, and the battle against them still continued. Indeed it was being fought against Israel in Hosea's day, and Israel too would be decimated in the same way for the same reason. It was as inevitable as what had happened at Gibeah.