What if Grindelwald defeated Dumbledore?

How could Dumbledore defeat Grindelwald since Grindelwald had the Elder Wand?

While the Elder wand is a very powerful wand, it isn't really unbeatable. Dumbledore confirms this in his notes on "The Tale of the Three Brothers" - the story of the Elder Wand shows that its owners have been defeated in one-to-one battles many times and that winning every duel is no guarantee.

“What any intelligent witch or wizard must notice while studying the so-called story of Elder Wand is that every man who claims to have possessed him has insisted that he is“ unbeatable ”when the known facts are beyond its passage through many owners are known 'hands show that it has not only been hit hundreds of times, but also causes trouble when Grumble the Grubby Goat attracts flies.'
- The stories of Beedle the Bard

There is another example in the history of the Elder Wand of a wizard who had him when was considered a powerful wizard and was defeated by another of relatively equal reputation. This seems directly analogous to what happened between Grindelwald and Dumbledore.

"All that is certain is that a wand that was owned by its owner Barnabas Deverill," Eldrun 27 Wall "appeared in the early eighteenth century and that Deverill used it to build a reputation as a fearsome warlock until his reign. The terror was ended by the equally infamous Loxias, who took the wand and renamed it the" death wand " and thus devastated anyone who displeased him.
- The stories of Beedle the Bard

Grindelwald with the Elder Wand was no guarantee that he would win every duel. The owners of the Elder Wand have been defeated many times. Having it would in no way guarantee him victory.