Alexander movie, Do You Want To Live Forever

It can be heard at 5:27:

https://www.youtube.com/watch?v=weOYBv5alHo

Apparently that was a line common during the Vietnam war in American soldier's mouth... Least I heard that somewhere

I think it was WW1. But why would Stone use it? It wasnt in his CONAN script that I recall, and is clearly a throwback to CTB for some reason. But the line could've been from the Spartans or maybe Alexander himself? I know both Stone and Milius have similar tastes, and do scribe lines from historical sources to use as dialogue.