מדרש אויף שמות 9:6

(Exodus 14:7) "And he took six hundred choice chariots": Whence came the horses required for the chariots? If you would say, from Egypt, is it not written (re the plague of pestilence, Ibid. 9:6) "and all the cattle of Egypt died"? And if you would say, from Pharaoh, is it not written (Ibid. 3) "Behold, the hand of the L–rd is in your cattle in the field, in the horses, etc."? And if you say, from Israel, is it not written (Ibid. 10:26) "And our cattle, too, will go with us; not a hoof will remain"? Whence, then, did they come? From those (of his servants) who feared the word of the L–rd (viz. 9:20) and drove his horses from the field into the houses. We find, then, that the cattle driven off by those who feared the word of the L–rd proved to be an impediment to Israel — whence R. Shimon says: "the best of the gentiles — kill! The best of the serpents — crush its head!"

Our sages of blessed memory said: The Holy One, blessed be He, followed the military tactics employed by kings when inflicting the plagues upon them. When the people of a province rebel, a human king dispatches his legions to surround them. First he dams up their water supply. If the people repent, well and good; but if not, he orders thunderous noises to be directed against them. If they are contrite, good; but if not, he commands that arrows be shot at them. If they relent (their actions), well and good; but if not, he sends barbarians against them. If they retract, good; but if not, he orders other reprisals to be taken against them. If they are contrite, well and good; but if not, he orders naphtha to be hurled upon them. If they are repentant, good; but if not, he catapults stones upon them. If they repent, good; but if not, he turns a large population against them. If they retract, good; but if not, he imprisons them. If they relent, good; but if not, he destroys their leaders. The Holy One, blessed be He, attacked Egypt with the tactics employed by a king. The first thing He did was to cut off their water supply, as it is said: And he turned their rivers into blood. They were not contrite, and He therefore brought tumultuous noises upon them. These were the frogs. R. Yosé the son of Hanina said: Their croaking was harder to bear than the havoc they wrought. Still they did not repent, and so he shot arrows at them. These were the gnats, as it is said: And there were gnats upon man, and upon beast (Exod. 8:13). They penetrated the bodies of the Egyptians like spears. However, they did not become contrite, and so He sent barbarians against them. These were the swarms, as it is said: And there came grievous swarms of beasts (ibid., v. 20). Still they did not relent, and so He took other reprisals against them. He brought the murrain: And all the cattle of Egypt died (ibid. 9:6). Nevertheless, they did not repent, and so He poured naphtha over them. These were the boils, as it is said: And a boil breaking forth with blains upon man and upon beast (ibid., v. 9). Still they were not contrite, and so He catapulted projectiles upon them. This was the hail. But even then they did not repent, and so he stirred up a large population against them, that is, the locusts. Even so, they did not mend their ways, and so He imprisoned them. This was the darkness, as it is said: And there was a thick darkness (ibid. 10:22). When they refused to repent, He killed their important men, as it is said: The Lord smote all the firstborn (ibid. 12:19).

Whatever the Egyptians devised against Israel, [the Holy One] brought against them. Those < Egyptians > devised for them to draw their water. What is written (in Ps. 78:44)? HE TURNED THEIR NILES INTO BLOOD SO THAT THEIR STREAMS WERE UNDRINKABLE. Those < Egyptians > devised for them to be laden as traveling traders.²²Gk.: pragmateutes. He sent frogs upon them and covered them with resin. Those < Egyptians > devised for them to work on the land. The Holy One made the land swarm with gnats. Those <Egyptians> devised that they should bear their children as a paidagogos.²³The Greek word designates a slave who was assigned to a child as an attendant and guardian. He sent lions, wolves, and bears among them. <If> an Egyptian had ten children, when he told Israel to bring them to the market place, the lion came and took one, the wolf one, the bear one, the leopard one, and the snake one. Then he came <back> to the Egyptian alone. The Egyptian said: Where are my children? Israel said to him: Sit down and I myself will render you an accounting: The lion took one <etc.>; and so <he continued> on, until he had given him an accounting. Now they (the Egyptians) devised that they would tend their livestock, but he sent out the plague among them, as stated (in Exod. 9:6): THEN ALL THE EGYPTIAN LIVESTOCK DIED. They intended that they would be bath attendants {bathe them}. He brought the boils upon them. They intended to stone them with stones. The Holy One brought the hail upon them, as stated (in Exod. 9:22): THAT THERE MAY BE HAIL <IN ALL THE LAND OF EGYPT >…. They intended that they would be their vine dressers. The Holy One brought the locusts, who ate their trees, as stated (in Exod. 10:15): AND THEY ATE ALL THE GRASS OF THE EARTH…. They intended to imprison them in prison. The Holy One brought darkness upon them, as stated (in Exod. 10:23): THEY COULD NOT SEE ONE ANOTHER…. They intended to kill them. The Holy One killed their first-born, as stated (in Exod. 12:29): THAT THE LORD SMOTE ALL THE FIRST-BORN. They intended to drown them in water. The Holy One also drowned them in water, as stated (in Ps. 136:15): BUT OVERTHREW PHARAOH AND HIS HOST IN THE REED SEA….