When and why was .45ACP ammo created?

1904. The American army ran into a few problems during the Philippine-American war. Mainly because .38 pistol rounds were completely unaffective against the Philippine warriors and the standard issue riffle rounds at the time also wouldn’t stop them. Colonel Thompson of the Infantry, and Major LaGarde of the Medical Corps conducted some tests (mainly against cattle) and decided that the minimum calibre round you needed against a detemnind foe was .45

The British army had had simmilar problems and invented the dum-dum.