This was in fact one of the most successful acts of covert US policy since WWII. One can speak of “private” military contractors, but in reality this was an operation conducted entirely by the US government.
The arming and training of Croat forces changed the strategic balance of the Croat/Bosnian war and, pretty much, ended it. The current state of peace and relative prosperity for, at least, the bulk of Croatia and Bosnia is a direct result.
Until that time the Serbs were militarily superior due to the open support of the Bosnian/Croat Serbs by the Serbian state. After this there was no choice for the Serbs but to negotiate realistic terms.
Except that the dead don't negotiate now, do they?