“These leaders of Simeon, registered by name, came in the days of King Hezekiah of Judah. They destroyed the tents and the Meunim who were found there. They exterminated them to this day. They settled in their place, because there was pasture there for their flocks.”
This incident is from the time of King Hezekiah who ruled Judah from 716-687 BCE as indicated in 2 Kings, chapters 18-20. There must have been a census of some sort as clearly the Simeon clans are part of Judah. They went after the Meunim, a people southeast of the Dead Sea. Supposedly they destroyed them and exterminated them. Thus they took their land as pasture for their flocks.