The 7.65 Parabellum (.30 Luger) was originally designed for the Luger pistol. The Borchardt, the predecessor of the Luger was chambered for a cartridge dimensionally identical to the 7.63 Mauser (.30 Mauser), but considerably less powerful.
When the 7.65 caliber Luger was tested by the German army, they liked the gun but wanted a bullet of at least 9mm. Luger, up to then thinking along the lines of cartridge support (headspace) using a case shoulder, found it was not possible to expand the 7.65 case to 9mm and still have a useful shoulder. So he hit on the idea of supporting the cartridge on the case mouth. Browning had gone another route, avoiding feed problems in a magazine by reducing the cartridge rims to a minimum and thus inventing a line of semi-rimmed cartridges (.25 ACP, .32 ACP, .38 ACP and 9mm Browning Long).
Apparently some time around 1904, Browning either heard about or saw a Luger with the 9mm cartridge headspacing on the case mouth, and the light went on. After that, Browning's cartridges headspaced the same way, but Browing went Luger one better by making his rimless cases (.380 ACP and .45 ACP) straight instead of tapered like the 9mm Luger.
Jim