I find stereotypes to be quite funny, so that's not what I find offensive about it.
What I don't like is how it makes the axis powers seem more like the protagonists while the allies were less competent. I also don't like how it portrayed WWII as a kawaii boy-love fest.
I wouldn't mind if Japan made an anime about WWII. Or even if it was a bit humorous (in similar light as M*A*S*H). But it was a time of disgusting war crimes and bloody battles. Fan-service has no place in it. Germany invading Poland was a BAD thing, not "Teehee buttsecks".