Following Ariana's death, Grindelwald flees Britain and eventually steals the Elder Wand, as we see in the movie Harry Potter and the Deathly Hallows: Part 1.
During this time period, Grindelwald rises to power, leading his followers to launch attacks across Europe and commit mass slaughters. The international wizarding community begins hunting him as he wreaks havoc in both the magical and Muggle worlds.