I've read a lot about the WW1 period in the U.S., and I don't believe I've ever come across a reference to another *extremely popular* pacifist song at that time.

Presumably others were written during the period of neutrality (1914-17), but I don't know of any that became hits.

During the Civil War various Copperhead songs were printed with lines like, "Abraham Lincoln, what are you about?/ Stop this war, for it's played out!" Their point was political, not ethical, and I don't think any of them could reasonably be called "hits." The idea of a Southern song protesting the war was virtually unthinkable.

And I'm drawing a distinction between songs that protest specific wars and songs like "I Didn't Raise my Boy" which oppose all war on principle.

You might want to look at a little book called "The Best Anti-War Song Ever Written," published by Mudcat's Dick Greenhaus. Until the First World War, even "Johnny, I Hardly Knew Ye!" was generally understood as humor.

(Pacifist *poems,* however, have existed since at least the 18th century.)