We all know that Dumbledore defeated Grindelwald without killing him (Voldemort killed him in Deathly Hallows). We also know that Grindelwald is only the second most powerful and dark wizard of all time, after Voldemort. Maybe because we only saw a brief glimpse of Voldemort reign fully in Death Hallows (he may have returned in Goblet of Fire but he didn't reign openly until Deathly Hallows) and we saw more of Grindelwald reign as it plays out in the Fantastic Beasts; I'm personally I'm more impressed with Grindelwald's reign. It makes me wonder how Dumbledore was able to defeat him and why he fell to second to Voldemort in the most powerful and dark wizard of all time. I have a theory.
What if Dumbledore defeated Grindelwald with.... love? In HP universe, love is suppose to the most powerful magic. While some like Voldemort may see it as simple magic, and maybe see the reason why Grindelwald simply defeated by it as the reason why he fell to second most powerful and dark wizard all time. Dumbledore always talk the talk about love, maybe we'll see he'll put the walk the walk in Fantastic Beasts 5 movie. So would everyone be okay with that? That how Fantastic Beast series will end, with Dumbledore defeating Grindelwald with simply love.