Allah tells us about the blessings which He gave His servant and Messenger Musa, the speaker, may the best of peace and blessings from his Lord be upon him, He revealed the Tawrah to him after He destroyed Fir`awn and his chiefs.
(after We had destroyed the generations of old) After the revelation of the Tawrah, no nation would again be punished with an overwhelming calamity; instead the believers were now commanded to fight the enemies of Allah among the idolators, as Allah says:
(And Fir`awn, and those before him, and the cities overthrown committed sin. And they disobeyed their Lord's Messenger, so He seized them with a strong punishment) (69:9-10).
(as an enlightenment for mankind, and a guidance and a mercy,) guidance from blindness and error. A guidance to the truth and a mercy means, to show the way towards doing righteous deeds.
(that they might remember.) means, that the people might be reminded and guided by it.