G-d tells Moshe to inform the Jewish People that He is going to take them out of Egypt. However, the Jewish People do not listen. G-d commands Moshe to go to Pharaoh and ask him to free the Jewish People. Although Aharon shows Pharaoh a sign by turning a staff into a snake, Pharaoh's magicians copy the sign, emboldening Pharaoh to refuse the request. G-d punishes the Egyptians and sends plagues of blood and frogs, but the magicians copy these miracles on a smaller scale, again encouraging Pharaoh to be obstinate. After the plague of lice, Pharaoh's magicians concede that only G-d could be performing these miracles. Only the Egyptians, and not the Jews in Goshen, suffer during the plagues. The onslaught continues with wild animals, pestilence, boils and fiery hail. However, despite Moshe's offers to end the plagues if Pharaoh will let the Jewish People leave, Pharaoh continues to harden his heart and refuses.
“And the sorcerers did thus with their spells, and the frogs arose on the land of Egypt.” (8:3)
When one reads of the plagues of Egypt one tends to think that nothing else was happening in Egyptian life at the time. However, the Midrash relates that during the plague of frogs there was an ongoing war between the Egyptians and their neighbors — the nation of Kush — over the precise line of the border.
When G-d commanded the frogs to swarm over Egypt, they did so only on the Egyptian side of the border, so further dispute became moot. However, the Egyptian sorcerers tried to create a little extra lebensraum by trying to make the frogs swarm over the borders on the side of Kush.
G-d frustrated their designs, as the verse says, “And the frogs arose on the land of Egypt…” Even the frogs brought up by the Egyptian sorcerers arose only “on the land of Egypt,” and not on the land of Kush.
Sources: Avodat B’Yehosef in Mayana Shel Torah