The United States signed the 1907 provision to the Hague Accords. It was ratified in late 1909.
The 1907 provision adopted language that was less specific than the 1899 pact -- "In addition to the prohibitions provided by special Conventions, it is especially forbidden: (e) To employ arms, projectiles, or material calculated to cause unnecessary suffering."
The 1899 pact:
"Hague accords Laws and Customs of War on Land (Hague II); article 23, July 29, 1899, also is rather vague in its reference to projectiles:
"To employ arms, projectiles, or material of a nature to cause superfluous injury;"
However, that's expanded (no pun intended) on Declaration III (
http://avalon.law.yale.edu/19th_century/dec99-03.asp)
"The Undersigned, Plenipotentiaries of the Powers represented at the International Peace Conference at The Hague, duly authorized to that effect by their Governments,
Inspired by the sentiments which found expression in the Declaration of St. Petersburg of the 29th November (11th December), 1868,
Declare as follows:
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."
While the United States did not specifically sign onto the pact that included the definitions, we embraced the spirit of those definitions in our approach to small arms ammunition... while using high explosives to rip enemy soldiers limb from limb.
The 1899 pact also says that its provisions are only binding in combat between two signatories. That appears to be the out that the United States is using -- a strict reading and interpretation of the definition.
Of course, there would be bitching about it from the rest of the world...
"What, the non-signatory, non-recognized, non-nation of "Lop off the Infidel's Heads with big Knives" is bitching that we're using hollow point ammunition? What part of "not a signatory, not recognized, and not a nation" don't you understand?"
Back in the 1960s and 1970s when police in the US started moving towards hollowpoint ammunition there were some rumblings in the international community about their use.
Response (not sure if it came from State Dept. or where) was generally "this is an internal matter. If one of your nationals breaks our laws and gets shot with a hollowpoint bullet... we'll we're not at war with you, are we? No? TS."
