Where did the term "Politically Correct" come from?

Truman to MacArthur in WWII

For the last six odd years, almost all of the things I wanted to write or say have been stymied by that modern term referred to as ‘POLITICAL CORRECTNESS’..

Although I consider myself fluent in English, that term was not in my vocabulary.

Curiosity got the better of me so I decided to do a little research.
After two weeks of chasing fruitless leads, I found what I’d been looking for at the Truman Library and Museum in Independence Missouri .

An unnamed source there sent me copies of four telegrams between then-President Harry Truman and Gen Douglas MacArthur on the day before the actual signing of the WW2 Surrender Agreement in September 1945.. The contents of those four telegrams below are exactly as received at the end of the war - not a word has been added or deleted!

(1) Tokyo , Japan
0800-September 1,1945
To: President Harry S Truman
From: General D A MacArthur
Tomorrow we meet with those yellow-bellied bastards and sign the Surrender Documents, any last minute instructions?
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(2) Washington, D C
1300-September 1, 1945
To: D A MacArthur
From: H S Truman
Congratulations, job well done, but you must tone down your obvious dislike of the Japanese when discussing the terms of the surrender with the press, because some of your remarks are fundamentally not politically correct!
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(3) Tokyo , Japan
1630-September 1, 1945
To: H S Truman
From: D A MacArthur and C H Nimitz
Wilco Sir, but both Chester and I are somewhat confused, exactly what does the term politically correct mean?
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(4) Washington, D C
2120-September 1, 1945
To: D A MacArthur/C H Nimitz
From: H S Truman
Political Correctness is a doctrine, recently fostered by a delusional, illogical minority and promoted by a sick mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end!

Now, with special thanks to the Truman Museum and Harry himself, you and I finally have a full understanding of what ‘POLITICAL CORRECTNESS’ really means