"Now y'all fight fair..."
On this date in 1899, the first Hague Convention was signed.
Most notable of that peace treaty was Article 23 of the REGULATIONS RESPECTING THE LAWS AND CUSTOMS OF WAR ON LAND, which declared that it was simply not allowable "To employ arms, projectiles, or material of a nature to cause superfluous injury."
This also led to the following declaration:
Now, you can shoot someone with a bigger and more powerful weapon, of course, but you just can't make it expandable. That would hurt your enemy too much. We here have the first documented case of an agreement to just shoot your enemy a little bit. Which, of course, is fallacious. Combatants from signatory nations could shoot shoot their enemies as much as they wish... just not with expanding projectiles.
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 no-expanding-ammo rule is often misattributed to "the Geneva Convention[s]." Just like hearing a 2nd grade tattle-tale girl declare that "you're not supposed to," and "you're gonna get in trouble, 'cause Teacher says you can't do that," you'll regularly hear people assert every rule of war as being enforceable to all nations, under "The Geneva Convention." [sic]
For reasons that I don't understand, the United States generally adheres to this provision of the Hague Accords, even though it wasn't a signatory power of it.
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.
Even if we were to abide by the Accord, we could use expanding ammunition when in hostilities against a non-signatory country.
Friend Stephen A Camp has convinced me that the 9mm would be a very fine self-defense option... if one were to use adequate expanding ammunition for it. That, however, is not the case for our servicemen carrying M9 pistols with 115g FMJ ammunition. What a shame that we impose such silly limitations even on our servicemen's defensive arm. One wonders why.