Here's an earlier song with the same title, and its connection to WWI is a bit more explicit.

My transcription from the sheet music at UCLA Archive of Popular Music:

MY BUDDY
Words by Byron Gay, music by Neil Moret
San Francisco: Daniels & Wilson Inc., 1919.

1. We've been pals since we were boys, my buddy.
Side by side we've seen each other through.
Sympathetic, strong of heart,
Comrades from the very start,
But the best of friends must part,
My buddy.
Don't try to speak; I understand.
You've done your share in No Man's Land.

CHORUS: Buddy, buddy, my best pal,
My buddy, buddy, buddy,
Must you leave me now?
Some day in a better land,
I will grasp you by the hand,
My buddy, buddy, good luck, goodbye, buddy.

2. Gee, it hurts to see you go, my buddy.
Heavy-hearted, I will miss you so.
You have always been so fair,
On the level, on the square,
Giving more than half your share,
My buddy.
They've got you, pal; gee, what I'd give
To take your place and let you live! CHORUS