Well, architects generally try to please their customers, but also try, at the same time, to make some of their Ideas alive. And so He did, he brought an idea to life while pleasing the people he worked for. I, personally don't belive that Mies was a Nazi. Besides, I haven't read anywhere that he spoke for Nazis or anything like it. Yes, He did work for them, but what percentage of architects wouldn't bring an idea, or a concept to life, if posible, then to say: "Hell, I ain't gona work for them, their politics is bad!"
No, Mies used the opportunity to make great architecture, like most of other architects would.
Mies was simply an excellent architect.