how did the stirrup revolutionize warfare in the early middle ages?

Answer 1

If you're talking about saddle stirrups, I can imagine it was a lot easier for men to stand in the saddles with a bow and arrow or wield a sword. Your balance is more easily maintained on a horse when you have stirrups. Of course, really disciplined horsemen wouldn't need stirrups for lift or balance, but we can't all be experts.

Answer 2

First the strirup was developed before the Middles Ages, it was developed in China approximately in the 4th century althought is may have been in use before then.

The person who answered already, is correct in that brace d the rider and revolutionized warfare on horses.