Exodus 8:15

But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the LORD had said.

This verse belongs to the plague of frogs, in which the Nile, an object of Egyptian worship, was turned into a source of pollution and torment to show that the river and its gods could not protect the people. It records what happened after the Lord answered Moses and gave relief by removing the frogs, despite Pharaoh's promise to let Israel go.

When Pharaoh saw the respite and his immediate distress and suffering eased, he hardened his heart again and refused to listen, breaking his promise and stubbornly refusing again to let Israel go, exactly as the Lord had said. His earlier miracles had not moved him because the magicians' imitations led him to believe the rod becoming a serpent and the river turned to blood were performed by magic, which left his heart hard and only increased his hatred of the Israelites. The manner of removing the frogs gave Pharaoh and all Egypt proof that vain philosophy could not explain the work away as magic but as a judgment from the God of Heaven. For God could have made the frogs vanish to dust in a moment but instead let them die where they were and be gathered into heaps, with their bodies seen before them corrupting the air.

Behind the hardening was a conflict of power, with God working through Moses not by human power but to convince Pharaoh that the great I AM had sent him to demand Israel's release to serve Him. While Satan worked through the magicians by the power of their god the Devil to counterfeit God's work, sustain Pharaoh's resistance, stagger the faith of Moses and Aaron, and keep Israel in bondage. God's work was shown superior to Satan's, for the magicians could imitate only a few miracles and then failed when Aaron at God's command stretched out his hand and the dust became lice throughout Egypt. So that they were compelled to acknowledge to Pharaoh that this was the act of God and could not even save themselves from the plagues, giving Pharaoh sufficient evidence to believe if he would, yet he remained unmoved and his stubbornness continued into further judgment and affliction.

Pharaoh's crying for removal was false mourning not unto repentance, regretting his defiance only because its results brought disagreeable circumstances and grieving over unexpected consequences rather than sin itself. Which has no merit to remove sin, left his heart unchanged and ready to repeat his crime when tempted. And illustrates that refusing God's invitations hardens the heart and leads to greater resistance to later invitations. Pharaoh stands as a wilfully obstinate warning, like Nebuchadnezzar, against imitating his stubbornness. For when a man exalts his own ways and judgment as supreme he comes under Satan's jurisdiction and is led blindfold by him, with spirit and methods conforming little by little to the deceiver until the whole mind is under a spell from which he has no power to escape.