In general, the most notable impact fascism had on the world was causing the WWII. One of the beliefs of fascism was extreme nationalism, in terms of eliminating other "inferior" nations and expanding territories. Hitler, the most notorious fascist ever, put the theories in practice and invaded Poland in 1939. sembra naturale?

This sounds really great! Here are some edits!

"In general, the most notable impact fascism had on the world was causing the WWII. "
There may be a better transition than "in general" but this sounds good.
Remove "the" in front of WWII.
This should be "The most notable impact fascism had on the world was causing WWII."

"One of the beliefs of fascism was extreme nationalism, in terms of eliminating other "inferior" nations and expanding territories."
"in terms of" doesn't make sense here. This sentence is confusing because it is unclear if the second half of the sentence is part of the "one belief." I'm not sure if you are trying to explain "extreme nationalism" or if you are saying "expanding territories" is caused by people being extremely nationalistic/ fascist.
Could be "One of the beliefs of fascism was extreme nationalism, which results in people wishing to of eliminate other "inferior" nations and expand their territory."

"Hitler, the most notorious fascist ever, put the theories in practice and invaded Poland in 1939."
"in" should be "into"
"the" is optional in front of "theories," in this case.
Could say "Hitler, the most notorious fascist ever, put theories into practice and invaded Poland in 1939.