The Dum Dum was the reason, but it wasn’t specifically named as banned...and it was worded so that the empires could continue to use the bullets in colonial wars against tribes and other non-European groups. Rather unsporting, what? “The Contracting Parties agree to abstain from the use of bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core, or is pierced with incisions. The present Declaration is only binding for the Contracting Powers in the case of a war between two or more of them. It shall cease to be binding from the time when, in a war between the Contracting Parties, one of the belligerents is joined by a non-Contracting Power. “ So, as you said, technically we weren’t/aren’t bound by it, and thus anyone we’ve fought against or allied with is also not bound.