The threat posed by Grindelwald and his plans for world domination became so dire that the Wizarding world clamored for Dumbledore to intervene. It was only when he had no choice that Dumbledore finally confronted his old friend in an epic duel. In the end, Dumbledore wins the duel and takes Grindelwald’s wand, the Elder Wand of Deathly Hallows fame. This battle is such a notable event that it is featured on Dumbledore’s Chocolate Frog card as one of the three things for which Dumbledore is particularly famous (PS6). Grindelwald is subsequently imprisoned in Nurmengard (DH18).
The spectacular final battle between Dumbledore and Grindelwald takes place in 1945, the final year of World War Two, and the year that Voldemort finishes Hogwarts.
Grindelwald remains imprisoned at Nurmengard until his murder by Voldemort in 1998 (DH23).
Rowling apparently planned for Grindelwald to have been killed in this epic duel. Here are her comments in an interview from 2005:
ES: Our other “Ask Jo” question [is] about Grindelwald, which I’m sure you’ve been gearing up for us to ask.
JKR: Uh huh.
ES: Clearly -
JKR: Come on then, remind me. Is he dead?
ES: Yeah, is he dead?
JKR: Yeah, he is.
ES: Is he important?
JKR: [regretful] Ohhh...
ES: You don’t have to answer but can you give us some backstory on him?
JKR: I'm going to tell you as much as I told someone earlier who asked me. You know Owen who won the [UK television] competition to interview me? He asked about Grindelwald [pronounced "Grindelvald" HMM…]. He said, “Is it coincidence that he died in 1945,” and I said no. It amuses me to make allusions to things that were happening in the Muggle world, so my feeling would be that while there's a global Muggle war going on, there's also a global wizarding war going on.
Interesting that Dumbledore, who we see as a peace-loving, gentle old man, was so revered by the Wizarding community that he was called upon specifically to be the champion to defeat this powerful Dark Wizard. One can't help but wonder if the world at large knew of Dumbledore's personal connection to Grindelwald and if that figured into their plea that he deal with his old friend. --SVA