The fact that G-d had “fortified Pharaoh’s heart” and thus gave him no choice but to force us to stay in Egypt is definitely puzzling. But as we’ll see, it will actually enhance our understanding of free will.
Rambam agrees that G-d had in fact tampered with Pharaoh’s free will, but he maintains that G-d had every good reason to, since it was all part of His overarching system of higher justice. For you see, Pharaoh sinned in many ways aside from enslaving us. As Rambam reminds us, Pharaoh oppressed us early on; as it’s said “he said to his people, Behold, the Children of Israel are more numerous and mighty than we. Come; let us deal cunningly (i.e., cruelly and maliciously) with them!” (Exodus 1:9-10). And so they committed all sorts of crimes and inhumane acts against our people, “freely … rather than by compulsion” Rambam underscores.
“So G-d punished them for that” Rambam asserts, “by preventing them from doing teshuvah”. For as Rambam put it elsewhere, G-d ordained that anyone who’d committed a horrendous sin or a great number of sins, as Pharaoh did, is punished by being prevented from doing teshuvah (Hilchot Teshuvah 6:3).
And so Pharaoh and his people could no longer realize the extent of their cruelties; no longer regret them; no longer decide not to follow through on them ever. So their free will was indeed taken away from him to that extent. Why? Because they’d gotten themselves so deeply mired in sin and cruelty that they’d lost their humanity; hence their free choice, which is so much of what defines our humanity, was rescinded in all fairness.
That’s the phenomenon that G-d was following through on when He “fortified Pharaoh’s heart”. G-d’s overarching justice demanded that reaction to such evil individuals, even though it undid their free will. The underlying point is that even though free will is fundamental to the human condition, it’s still and all out-ranked by other factors.
Now, there’s something to be said about this little-known phenomenon that hits very close to home and should stun anyone in search of spiritual excellence. It’s the whole idea that there are times when our lapses get to be so numerous and so “normal” for us that we can be prevented from getting a handle on them and doing teshuvah! Hence the lesson is that it’s vitally important for us to be aware of our behavior patterns and to stop any destructive ones as soon as possible.