In prophecy something that will happen in the future is described as something in the past. This emphases that the prophecy certainly will happen. Alternate translation: "he will strike them with his powerful hand."
## their corpses are like garbage in the streets ##
Isaiah compares their dead bodies to garbage. This emphasizes that many will die but no one will be there to bury them. Their bodies will be left to rot in the streets. The word "garbage" can also be translated as "refuse" or "manure." (UDB) (See: [[rc://en/ta/man/translate/figs-simile]])