Honestly, visiting Germany was for many wizards and witches a very strange experience, and it all was connected at the basic level with the very man he was visiting. Basically speaking, thanks to Gellert Grindelwald, both parts of Germany no longer had any form of true separate wizard culture to speak of. It all was due to various factors in Grindelwald's rise and the war itself.
While many people knew that Grindelwald had been a magical supremacist who believed that those with magic should rule over those without - essentially turning them into slaves - many overlooked that Grindelwald was not blind to the realities. Essentially, he'd already known how far the non-magical world had come and that any grandiose plans for absolute domination were mere dreams. The numbers alone strictly spoke against it - any open attempt in that direction would end in disaster.
That didn't mean however he'd shelved his plans, he merely adapted them to reality. He also was a very pragmatic man and had known that sometimes you had to make compromises in order to make the first steps. So in order to gain control over the whole magical population of Europe, he needed allies. Thus he had done the exact opposite of what the end result of his philosophy dictated - he reached out to allies, magical and non-magical alike.
On the magical side, he found allies in the pure-bloods of his country. The massive turmoil that had shaken Germany ever since 1914 had bled through to its magical community - after all, full separation was an illusion - causing them to become very insecure. Even worse, the growing number of muggleborn and half-bloods had started to feel like a threat to them. They had felt Grindelwald was the right man to deal with the problem.
On the non-magical side, Grindelwald had seen where the wind was blowing in the early 30s and thus came into contact with the Nazis. Impressed by the sheer drive for power of their leader, Grindelwald forged a dark pact with Hitler, in that Grindelwald would help to destroy all magical opposition in the war that would surely come, while Hitler promised to help Grindelwald with his allies' problems.
Grindelwald himself had not been a blood-purist, but knew he had to appease his magical allies. He had seen the concentration camps as ideal for not murdering muggleborn and half-bloods, but destroy their spirit until they could be used essentially as a magical slave caste. There had been enough pure-bloods very willing to do the dirty work in the camps.
Of course, this also was the root of the current situation.
When they were getting crushed from all sides at the end of the war, the pure-bloods realized they would lose everything, so they felt the world after was no longer worth living. They'd murdered all of their own children, before suicidally fighting to the death. Thus, the pure-bloods got all killed and the traumatized surviving muggleborn and half-bloods felt repulsed by a magical culture that had done that to them.
Dumbledore shook himself out of his thoughts when reaching the doors to where he would see Grindelwald. The two guards at the door could very well have worked in an ordinary prison, there was no outer difference, apart from wands instead of guns hanging from their belts. It represented how a separate magical culture essentially no longer existed in this country.
"Er wartet drinnen auf Sie," one of the guards said, opening the door for Dumbledore.
Dumbledore's first thought when seeing Grindelwald, was, that the man had grown old. Granted, almost 40 years in prison couldn't be good for anyone, but the contrast to the last time he'd seen him was massive. Grindelwald looked sickly; he had lost lots of weight and next to all of his hair. The biggest difference however was in his eyes; the once proud and arrogant man was no more, in its place now being a strange sense of calm peace.
"Hallo, Gellert. Ich wünschte ich könnte sagen, dass du gut aussiehst, aber das wäre eine offenkundige Lüge," he greeted his former friend.
Grindelwald gave him a weak laugh, surprising Dumbledore, who had expected the man to have become very bitter. "Oh, vielen Dank für die Blumen." Grindelwald stopped for a moment. "Thanks for greeting me in German, Albus, but I think you'll be more comfortable with your own language." His English sounded a bit slow, probably from lack of usage.
That was a surprisingly friendly greeting, considering the last time they met face-to-face was in battle. "For being a prisoner in the very prison you built for your enemies, you are in good spirits."
"Surprised? Well, all these years have changed me quite a bit. But enough of myself, what gives me the honor of your visit? It must not have been easy to come here, after all," Grindelwald asked, being curious. The man clearly enjoyed the day being different.
"Honestly? Recently I've been thinking a lot about the past and how it shaped me into the man I'm today. You played a big part in it, so I felt to truly understand, visiting you could be beneficial," Dumbledore explained.
"I harbour the guess you want to leave out...?" Grindelwald started to ask.
Dumbledore looked pained. "Yes. I'm not proud of that day, and I guess neither are you."
That had been the worst day in his life, when the man he'd thought of more than a friend betrayed him, resulting in his sister dead and his brother being alienated. That very day had shattered the arrogant man he had been, and transformed him into the man he was now. A man, who had learned a very painful lesson of how wrong he had been.
I can't be trusted with too much power. Look what happened. Had this not happened, I would have turned out like Gellert. That was the lesson that had stuck.