Russia fell out of Mongol control about 500 years before World War II, exactly what did calling someone "mongolian" mean in 1945 and why and how did this connotation come to be? Also how did it come to be associated with the russians, a people removed from Mongol Rule for more than 500 years?
I assume it is related to the Mongols reputation for savagery, and or the rule of the Golden Horde over the Russian principalities from the 1230s to the mid 1400s but I am interested as to the specifics.
You might be overthinking it. Patton would use "Mongol" to describe people that modern Americans would describe as "Asian," similar to how he would use "Negro" when modern Americans would use "black" or "African-American."
Here's an example of how ethnicity was categorized in the 19th century when Patton was born. So to answer your question, calling someone Mongolian or mongoloid in 1945 means something similar to calling someone "Asian" today.
Of course, most people in 1945 would ascribe traits such as deviousness or inscrutability to someone described as "mongoloid" or Asian. Patton does so in the passage you're referring to.
The difficulty in understanding the Russian is that we do not take cognizance of the fact that he is not a European but an Asiatic and therefore thinks deviously. We can no more understand a Russian than a Chinaman or a Japanese and, from what I have seen of them, I have no particular desire to understand them except to ascertain how much lead or iron it takes to kill them. In addition to his other amiable characteristics, the Russian has no regard for human life and is an all out son of a bitch, a barbarian, and a chronic drunk.