Summary: Thus was Midian subdued before the children of Israel. The Midianites were humbled and brought under the authority of Israel by the hand of Gideon; their power over Israel was broken, and the Israelites were liberated out of their hands.
Gideon Kept the Country Quiet for Forty Years [Judges 8.28]
28 Thus was Midian subdued before the children of Israel, so that they lifted up their heads no more. And the country was in quietness forty years in the days of Gideon.
28 Thus was Midian subdued before the children of Israel, so that they lifted up their heads no more. And the country was in quietness forty years in the days of Gideon. (Judges 8:28; KJV)
28 The power of Midian was crushed by the people of Israel, and Midian never again became a threat. So the land had peace for 40 years during Gideon's life. (Judges 8:28; GW)
Thus was Midian subdued before the children of Israel. The Midianites were humbled and brought under the authority of Israel by the hand of Gideon; their power over Israel was broken, and the Israelites were liberated out of their hands. It is evident from our study of Gideon that the invasion of the Arab hordes into Canaan was as shocking and desolating as the explosion of the Huns into Europe. It was the severest affliction ever inflicted upon Israel; and both it and the deliverance under Gideon lived for centuries in the minds of the people (Psalms 83:11).
so that they lifted up their heads no more; this was the end-of-the-line for Midian. From now on they would not act toward Israel, or any other nation, in a proud and haughty manner to insult them, and in a hostile way to invade and oppress them. They had received such a blow that they could not recover, and we do not read of any effort of theirs in the future to attempt or instigate a disturbance involving another nation.
and the country was in quietness forty years in the days of Gideon; that is, the land of Canaan; it was free from wars with Midian, or any other people, and enjoyed undisturbed peace and tranquility. According to Bishop Usher, this was the fortieth year from the rest restored by Deborah and Barak; and, according to Abarbinel and others, these forty years are to be reckoned from the beginning of the servitude; that is, the seven years' of oppression under the Midianites are included in them, but I cannot see that happening in this instance, and I do not know of any scripture to confirm this speculation. I believe that years of bondage cannot be counted with years of peace and prosperity. The true sense seems to be, that after the Israelites had been in subjection to the Midianites for seven years, and Gideon had delivered them, that from that point forward they had rest and quietness for forty years, which in all probability was the time Gideon lived after his victories.
This is the last period of peace recorded in the Book of Judges. The subsequent activities of Jephthah and Samson did not seem to produce an interim period of peace or delay the nation’s decline.
 (Psalms 83:11) “Make their nobles like Oreb, and like Zeeb: yea, all their princes as Zebah, and as Zalmunna.” There was a conspiracy of many nations against Israel, but Israel responded by appealing to the precedent of past deliverances, the psalmist prays, Do unto them as unto the Midianites, probably an allusion to the days of Gideon (cf. Jud 7:19–25; 8:1–7), as to Sisera, as to Jaban, at the brook of Kison, where God raised a tiny brook into a raging sea and swept away Israel’s enemies in a sudden torrent. Make their nobles like Oreb … Zeeb … Zebah … Zalmunna. Oreb and Zeeb were the ringleaders of the Midianites and were slain at a rock and a winepress by the Ephraimites (Jud 7:25). Zebah and Zalmunna were the kings of Midian who were captured and slain by Gideon himself (Jud 8:21). They fell by the hands of God when they attempted to rob God.