"arms...discourage and keep the invader and plunderer in awe, and preserve order in the world as well as property. ...Horrid mischief would ensue were (the law-abiding) deprived the use of them."
-- Thomas Paine
The average response time in one study of a large city in Texas in 2004 to a class 3 burglar alarm was 32 minutes. Response time, on average, for 911 calls today? 10 minutes.
The time it takes a bullet to reach the meth addict with a bad attitude and a knife kicking in my front door?
1200 feet per second.
If I can avoid danger without conflict while on the street I will do so, removing myself from that which might pose a threat, before it does. But the only time I am going to run from my own home is if the Crème Brûlée meets acetylene torch Christmas party dessert gets out of hand or if the house is flooding.
It's known as Castle Doctrine and it can vary state to state. If you are law abiding and intend to stay that way and keep a firearm in your house, get to know the law for your area. They can be complicated but it's still better protection than unicorns and a moat.