Probably the biggest mistake Pharoah made, was when he asked the wrong question: Who is Hashem that I should let the people go? I don’t know of any such Hashem.
Parshat וארא seems to be an answer to Pharoah that he was going to “eat those words.” After every plague, we are told, that this particular plague was in order that you know that there is a G-d in Israel.
In actuality, Hashem’s demonstration of His awesome might was meant for the Jewish people for all time. Twice daily, we mention יציאת מצרים in our prayers. We must also never forget that we must know Hashem.
The Rambam emphasizes that this idea of לדעת את ה׳, to know Hashem, is a fundamental principle of Judaism. We are to live with a constant awareness of Hashem’s Presence.
We are also meant to try to understand His ways to the best of our abilities. So it wasn’t only Pharoah who needed to know Hashem. It is a task for all the Jewish people and all mankind.