Given the choice, most people would much prefer to go through their entire lives without experiencing a violent confrontation. However, when a criminal threatens you and/or your family with the clear intent of doing harm, you are left with the choice of either meekly accepting whatever action the criminal decides to take or, defending yourself. Thus, given that choice, most people would choose to defend themselves. However, most people do not have either the time or the interest necessary to become a martial arts expert and that is why many people choose to carry a handgun. However, many shooters are intimidated by the noise and recoil a handgun generates and thus, they need a cartridge that has enough power to stop an assailant but not one so powerful that it’s difficult to shoot. Thus, the 9mm Parabellum is the best possible compromise between recoil, stopping power, and frame size.
Why a 9mm and not a .380 or a .45 you might wonder, well the answer to that question is because a .380 is underpowered and a .45 generates too much recoil for some shooters; especially those who are not experienced in the shooting sports. Thus, the 9mm Parabellum bridges the gap between those two cartridges by shooting a bullet that is the same diameter and weight as the .380 ACP (All Colt Pistol) but smaller in diameter and of lesser weight than the .45 ACP. Thus, it has enough kinetic energy to do significant damage to an assailant but not so much that the recoil makes it intimidating to shoot. Also, because the 9mm Parabellum is the single most popular self defense cartridge around the world, ammunition is readily available.
In addition, because it is the official military cartridge in many countries including the U.S., military surplus ammunition is often available at a significant discount; thus making frequent target practice economically feasible. Furthermore, there is an extremely wide range of both bullet weights and loads to choose from ranging from ultra light bullets to extremely heavy bullets and moderately slow loads to ultra fast loads. Plus, you can choose between Full Metal Jacket bullet construction for reliable functioning or various types of hollow points for limited penetration and superior stopping power or frangible bullets which are designed to essentially explode on impact for devastating stopping power combined with very limited penetration for use inside of your home where a missed shot could possibly endanger family members or even neighbors. Thus, you can choose the level of stopping power that you prefer by tailoring your choice of ammunition accordingly.
Then, there is the matter of frame size. For instance, the .45 ACP is a relatively powerful handgun cartridge and thus it generates a significant amount of recoil. Therefore, firearms manufactures must make the frame of a .45 ACP robust enough to withstand the repeated recoil generated when firing multiple rounds. Also, the .45 ACP fires a relatively large diameter bullet and thus, the frame of the .45 ACP must be large enough to hold the rounds contained in the magazine. However, because the 9mm Parabellum generates significantly less recoil and fires a significantly smaller diameter bullet, the fame size of the 9mm can be made significantly smaller which makes it far easier for many shooters to grasp and shoot.
Therefore, because the 9mm Parabellum bridges the gap in both stopping power, recoil, and frame size between the .380 ACP and the .45 ACP along with the fact that a wide range of ammunition is readily available as well as the fact the frame of the 9mm is necessarily smaller than that of the .45 ACP, you can easily see why a gun chambered for the 9mm Parabellum is the perfect first handgun for self defense purposes.