Capitalize first letter after colon?

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I don't recall ever seeing this (except for proper noun or acronym), but apparently it's an acceptable practice in "American English".

https://www.theatlantic.com/international/archive/2018/05/trump-north-ko...

"But the Trump administration may have little choice except to oversell the summit for another reason, this time of its own making: Before actually booking a Korean success, it committed itself to a second confrontation, against Iran."

Semi-colons, colons, and dashes - The Writing Center

https://writingcenter.unc.edu/tips-and-tools/semi-colons-colons-and-dashes/

 

Here's our suggestion: generally, the first word following the colon should be lower-cased if the words after the colon form a dependent clause (that is, if they could not stand on their own as a complete sentence). If the following phrase is a complete (independent) clause, you may choose to capitalize it or not.

 

If the following phrase is a complete (independent) clause, you may choose to capitalize it or not.<<<

Then why not simply start a new sentence?