Deuteronomy 32:36
For the LORD shall judge his people, and repent himself for his servants, when he seeth that their power is gone, and there is none shut up, or left. For the LORD shall judge his people, and repent himself for his servants, when he seeth that their power is gone, and there is none shut up, or left.
The verse belongs to the song Moses was commanded to teach Israel, preserving the record of God's dealings with His people for later generations. That song was to stand as a continuing sermon to warn, restrain, reprove, and encourage the nation. It recounts God's dealings with Israel and foreshadows future events. Within the song this promise comes after the account of Israel's unfaithfulness and coming calamity, followed by God's vindication of His people and judgment on their enemies.
It means the Lord shall judge His people but relent and have pity for His servants when He sees their power and strength are gone. They have become helpless, with none shut up or left and none remaining to help, escape, or endure. When they repent of their sins, He sees their helplessness and responds with compassion rather than punishment.
The song foreshadows future events, including the final victory of the faithful when Christ comes again in power and glory. The verse is applied to the South as a people being punished.