...a dog.
A dog can provide warning, no matter its size, in the form of an auditory alert. Certain dogs can provide a visual deterrent (normally your larger breeds traditionally bred for guard work). Some dogs are actually bred for defense.
I've actually almost crapped my pants when a dog alerted on me. I've probably shared this story before. We were going to pick up our German Shepherd at a home near Houston. It was dark outside, and as I approached the unfamiliar front door, I noticed that the door was open, but there was a glass door that was closed, allowing me to peer into the hallway. As I approached the doorway, a black shape with a roaring, menacing bark was headed full-speed for the glass door, right towards me. I must have jumped back 10 feet.
That menacing dog was the dog we adopted. A female German Shepherd.
Dogs are great companions, and ideally are literally family members. Dogs that are bred for work and guarding, traditionally, are among the smartest of all dogs.
Are dogs effective in "end of the world" scenarios. Yes and no, I would say. FERFAL teaches us that in Argentina, dogs were commonly killed with poison. It's not hard to shoot a dog either.
Having said that dogs have held value to humans for thousands of years, even though the cost has been significant (another mouth to feed).
Other issues to think about
: some insurance companies in some states will not sell you home insurance if you own "banned" breeds. You should research this before obtaining/purchasing a dog.