So, to start let's talk about how the swastika was discovered. In 1868 German archaelogoist Heinrich Schliemann embarked on a journey to discover the city of Troy using Homeric texts as clues. In 1871 he discovered Troy in Turkey, but also discovered that Troy was built upon several previous cities. As they tore down the various layers, Schliemann and his crew continuously discovered the swastika symbol at every layer. This being an age of discovery, people became super interested in trendy stuff including crazy archaelogical finds. So the swastika became a popular symbol especially in the US as a symbol for luck or eternity.

Around the same time, linguists were coming up with a theory of proto Indo-European cultures known as Aryanism. The idea being that similarities between Germanic, Romantic, and Sanskrit languages indicated that there once was a race of peoples who colonized Europe and India and changed over several centuries into the separate races. This race became referred to as the Aryan race by scholars. The swastika being discovered in several layers of ancient cities at the very intersection of Europe and Asia led to the swastika becoming associated with Aryanism.

Then a raging white supremacist French aristocrat named Arthur de Gobineu began writing that the Germanic and Nordic races were the purest current form of the Aryan race and that the Romantic and Indian races were Aryans who suffered impurities as they mixed with other races. As nationalism was growing in Europe especially in the wake up Germanic unification, Aryanism became more and more prominent in German society and as such, the swastika became a symbol of Germanic purity and superiority. Hitler wasn't even the first German purity group to use it. The Reichshammerbund adopted it in 1912 and the Bavarian Freikorps adopted it in 1918.

Sprinkle in a little bit of Nietzche's Ubermensch philosophy and suddenly you have a prophecy which states that one day the perfect race of men will rise and subvert all other races under there power, and that race must happen to be the purest form of the once great and mighty Aryan race which once conquered the world before becoming impure. Then Hitler shows up, throws the symbol on a flag like many did before and the rest is history.