In modern popular English Nazi is pronounced with a hard Z, almost "naht'zee," but in Churchill's "darkest hour" speech he pronounces it much more softly and as a single syllable, like "nahzee." Is the hard Z pronunciation a product of the anglicization of the word, was Churchill pronouncing it incorrectly, or has the pronunciation changed over time?
I can at least provide some background on the original pronunciation. The key component is understanding the German letter Z.
While in American English we call this “Zee”, in German it is called “Zet”. Unlike our pronunciation of the letter, this is pronounced “Tset” where the hard T and the S are pronounced almost together. This translates to an original German pronunciation of “Naht-see” with no trace of the English zzz sound.
In other words, any pronunciation involving the “zee” or “zzz” sound is inherently Anglicized, whether with the hard z or soft z as you have described.
Listen to this video talking about the usage of the words bezahlen vs zahlen. https://youtu.be/tp5khoPYkws
In German it’s actually the letter S which is typically pronounced like the English Z as you’ll hear several times in this video with the word “sagen” or “sag”.