Shemot 14:1-3 record that G-d had the Jewish people travel in a circuitous course when they left Egypt to make Pharaoh think that the Jews were lost in the desert. 14:4 then records that G-d told Moshe that He would harden Pharaoh's heart in order that the Egyptians would chase after the Jewish people. Afterwards, 14:5-7 record that once Pharaoh heard that the Jewish people had run away, he immediately wanted to enslave them again. Pharaoh gathered his army and began to chase the Jewish people. 14:8 then records that G-d hardened Pharaoh’s heart that Pharoah chased after the Jewish people. Similarly, Shemot 14:17 records that G-d told Moshe that He would harden the hearts of the Egyptian forces to chase the Jewish people. Note that this hardening of the heart is a little different than by the plagues since there the hardening of the heart was not to let the Jews leave, while here the hardening was to chase the Jews.
These verses raise two somewhat contradictory questions. One, once G-d was going to harden Pharaoh's heart to have him chase the Jewish people, why did the people have to march in a circuitous course? Two, why did G-d have to harden Pharaoh and the Egyptian forces heart since 14:5-7 record that they went to chase after the Jewish people before G-d hardened their hearts (in 14:8)?
Maybe the circuitous route was the way to show Pharaoh's true colors, when his heart was not yet hardened, that he was evil, even after experiencing the ten plagues. Even though 14:5-7 do not relate Pharaoh’s decision to chase after the Jewish people to the people getting lost in the desert, undoubtedly, he learned this information (see 14:3) and then he thought that G-d had only helped the Jewish people temporarily with the plagues, so he thought he had a chance to enslave the people again. Thus, his request for G-d to bless him, 12:31, is seen as a lie.
Why then did G-d harden Pharaoh’s heart one Pharoah was already chasing the Jewish people? The answer is that Pharaoh's desire to chase the Jews and enslave them was his intuitive response to hearing that the Jewish people were trapped (system one of the thinking process of the brain). However, after experiencing the ten plagues, a little more thinking (system two of the thinking process of the brain) would have made Pharaoh realize that he should just let the Jewish people go. Thus, 14:4 tells us that G-d would harden Pharaoh heart and 14:8 tells us that G-d hardened Pharaoh's heart that his thoughtful second system did not overcome his intuition response to chase down the Jewish people, see our discussion on 9:34,35 "A hard heart: System one and system two."
With regard to Pharaoh's soldiers, their hearts were initially not hardened to show, like Pharaoh, their cruelty that they wanted to attack a defenseless innocent people. This explains why their deaths at Yam Suf were just. However, when they saw that G-d split the Yam Suf, then they should have been sufficiently awed (even their system one response) to stop in their tracks and not go into the Yam Suf, so then G-d hardened their hearts to chase the Jewish people in the Yam Suf, where they died, 14:17, 28.