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They Shall Not Grow Old (2018) Full Documentary
(odysee.com)
It's been awhile but I still remember the part when recruiters were telling underage teenagers they needed to "step outside and have a few birthdays first". As in, 'come back and tell me the right age'.
Also, the part where one vet talks about taking prisoners, and how machine-gunners were not taken prisoner.
I knew that Tolkien had served in WWI, but this is the documentary that laid bare the origins of many aspects of Middle-Earth.
https://en.wikipedia.org/wiki/The_Great_War_and_Middle-earth