Part of the Nazi ideology was built on the idea of the Aryans, who entered Northern India perhaps in the early Iron Age. Their language and religion came to dominate most of the sub-continent. What we now call the Hindu religion and the Sanskrit language (the Latin of Indian languages) were from this group, the Aryans.

Germany had been the leader in Orientalism for many decades before the rise of the Nazis. This fine historical tradition was hijacked and used to find excuses to support the racial theories that Hitler and others were preaching.

Though the origins of the Aryans is still disputed, the leading modern theory is that they came from Iran (the word "Iran" shares origins with Aryan). But the German orientalists began to theorize that the Aryans were, well Aryans. The so-called master race must have traveled to India and subjugated the inferior dark-skinned people there, went the theory of the racially-charged orientalists.

The word "caste" literally means color. It has been theorized that the Aryans put themselves on the top, with the natives beneath them and this led to the modern caste system (that stubbornly holds on in many parts of India today). The Nazis naturally found this to be a neat story. Hitler chose to embrace the term "Aryan" for his beloved blue-eyed folk because of a feeling of connection with these half-legendary conquerors.

Now, as to the swastika. It is a symbol used by Hindus as well as the related religion of Buddhism. Hitler saw it as a lingering symbol of his ancestor's conquest in India, another remnant left behind. Just as Hitler was (in his mind) reinvigorating the modern Aryan, he was resurrecting what he saw as an ancient symbol of the Aryan.

The Nazi leadership was aware that the symbol was used elsewhere. It was seen as proof that "Aryans" are great and had a huge influence on the rest of the word.