For the scripture saith unto Pharaoh, Even for this same purpose have I raised thee up, that I might shew my power in thee, and that my name might be declared throughout all the earth.
Note 9 at Ro 9:17: Some people have taken this word from God about Pharaoh and made a paragraph out of it. They have drawn conclusions that God predetermines everything in people's lives to the degree that free will doesn't exist. That is not what the Lord was speaking of here.
We can be assured that Pharaoh had already had ample opportunity to respond to God prior to the time that God began to harden his heart. Since Pharaoh had already made his choice, even to the point that he proclaimed himself to be a deity and commanded the Egyptians to worship him, God was not unrighteous in bringing him into judgment for this.
God did not make Pharaoh the way he was, but God used, for His glory, the way Pharaoh had chosen to be. God exalted Pharaoh and gave him leadership of the nation, knowing full well how he would respond to His demands to let His people go. Since Pharaoh had already hardened his heart toward God, God was not unjust in continuing to harden his heart further until His glory was manifest completely.
This verse is depicting God as using Pharaoh's hardened heart for His glory, but Pharaoh had already had his chance. God simply upheld his choice and received glory through His triumph over Pharaoh and all his host.