In the time of Moses, Egypt oppressed Israel terribly. God punished Egypt for this. In the book of Exodus, he is clearly shown as the true God. Despite this, God's people still turned to the same nation who oppressed them for help in times of crisis. They needed to be reminded of God's sovereignty.
1 An oracle concerning Egypt. Behold, the LORD is riding on a swift cloud and comes to Egypt; and the idols of Egypt will tremble at his presence, and the heart of the Egyptians will melt within them.