Germany invaded many countries during World War II. Why didn't Germany invade Sweden during WWII?
Because there was nothing to gain by invading Sweden and much to loose if doing so. Sweden was a major source for iron ore for Germany and via neutral Sweden, Germany could exported other materials from countries which didn't have any relation with it including the allies countries.
I always thought that Sweden had a lot of aluminum which helped Germany in producing airplanes..., anything that was light and would enable it to travel fast.