They believed that pure blooded Indo-Europeans created civilizations which declined as they mixed with inferior races (like ancient Greeks or Romans).
Explanation please. Are you saying the Indo-Europeans declined like ancient Rome?
Romans ie Latins were Indo-European same way as Greeks were. But after they created their civilizations they absorbed people of other races. The decline was supposed to take long time measured in centuries or in generations, especially if the ruling group preserved racial purity.
Nazis were seeing Germanic race (Germans, English, Scandinavian) as superior because they preserved their Indo-European bloodline relatively intact. BTW, Hitler believed that English were superior to Germans and that Germans were doing well because they had superior leaders :)
Nazis have seen the successes of US as expression of the racial quality of the ruling WASPs. They have seen the expansion of Russia as a result of Germanic origin of Russian aristocracy and bureaucrats (imported by the post-Petrine Tsars).
Interestingly the later view led to the major miscalcalculation - Hitler was convinced that Soviet Russia after the Slavic peasant and Jewish takeover was not capable of resisting German army.