The southern portion of the kingdom had been overrun by Midianite hordes; the enemy with whom the Israelites had to contend on Moabite soil was Midianite and not Moabite.

Those who corrupted Israel on the high-place of Peor were Midianites in race.
The Midianites seem to have continued in occupation of Moabite territory for several generations.

Reuben was enabled to pasture his flocks in peace in its valleys, and it is probable that it was not till Hadad, the King of Edom, "smote Midian in the plain of Moab" that Midianitish supremacy came finally to an end.

It may be that Gideon's success against the Midianite oppressors of Gilead was one of the results of their overthrow by the Edomite prince.
At the same time, Midianitish supremacy did not mean the destruction of the Moabite kingdom.

Moab was still governed by its own kings, tributary vassals though they were to the foreigner.