The American military has been voting by mail since the Civil War. Abraham Lincoln insisted on it, even though there was a great deal of resentment towards Lincoln amongst the Union rank and file. He knew every single American citizen deserved the right to be counted and even if it hurt his chances, it was the right thing to do.
That’s just one reason why Abraham Lincoln is on Mount Rushmore.
https://www.nbcnews.com/politics/2020-election/how-do-you-know-voting-mail-works-u-s-military-n1186926