The book and movie "We Were Soldiers Once" takes its title from a line in a poem that runs, "We were soldiers once, and young" but I can't remember the name of the poem or the poet. Anyone know?

I found this quote from Joe Galloway but have been unable to find the poem:

We started off with the short list of about ten titles that we had mulled over for months. One of them was simply, Soldiers Once. The title is from the last line of an A.E. Housman poem about dead young soldiers. Then we thought we could use the whole damn line: "We were soldiers once and young." It resonated.
https://groups.google.com/g/alt.quotations/c/WIfuBg9EaK0