Exodus 3:19

And I am sure that the king of Egypt will not let you go, no, not by a mighty hand.

God gave Moses advance assurance beforehand that the king of Egypt would refuse to let Israel go. The king meant was the one then reigning, not the earlier king who had sought Moses' death. That refusal would persist even when confronted by divine power, by a mighty hand, so that Israel's deliverance was to meet determined resistance. Moses would have preferred to deliver Israel by leading them in war as their general.

The forewarning was given so that the certain refusal would not cause Moses to lose courage, that his courage should not fail. Pharaoh's resistance was the appointed occasion for God to manifest His signs and wonders before the Egyptians and before His people, smiting Egypt with all His wonders. By that display God would be magnified, teaching both Israel and the Egyptians that there is a living God who has power to save and to destroy. Israel was to be delivered by God's own mighty power for His own glory rather than by warfare. After the wonders, after He had smitten Egypt, Pharaoh would let the people go.