Trained dogs historically have been used in military and police work. The military uses them in combat and as scouts, sentries and trackers. Police use them to detain, pursue, and identify suspects, to detect illegal substances, deter crime, protect officers, and control crowds. (See Dog Bite Law, Police Dogs.) They also have been used historically in the protection of livestock. (Bryce Reece & Bonnie Brown, American Sheep Industry Association's Recommended Best Management Practices for Livestock Protection Dogs.)
Protection is also a dog sport. The best known protection dogs are referred to as Schutzhund dogs. "Schutzhund" is German for "protection dog." Schutzhund dogs are trained in tracking, obedience and protection work, and are tested as companions to people as well as for endurance. "The most important criteria for the assessment of protection work are ... [w]ell balanced drives[, s]elf-confidence[, a]bility to work under pressure; toughness; resilience[, s]teadfast, sound nerves[, and w]illingness to take direction (commands), responsiveness to the handler." (United Schutzhund Clubs of America, USCA Working Dog Trial Rule Book, page 47, accessed 2/15/15.) There are three IPO trials and the dogs are rated as to which they pass.
To see a well-done IPO trial: Kingsont Martin vom Bullenfeld - 2014 USRC National IPO Champrionship IPO3 Protection (95 points SG), https://www.youtube.com/watch?v=jSzpE4VmRQk accessed 2/15/15. The rules for Schutzhund dogs are public. (United Schutzhund Clubs of America, USCA Working Dog Trial Rule Book, ibid.)
The protection dog industry has expanded its market to the general public. High-end protection dogs can be purchased for $40,000 to $60,000, with some dogs selling for hundreds of thousands of dollars. (NY Times, For the Executive With Everything, a $230,000 Dog to Protect It.)
Civilians who purchase these dogs expect them to be fierce defenders of the family and household while also being cuddly with the owners and their kids, and friendly to guests. See how the dogs are marketed to the public by doing an Internet search using the term "protection dog." The dogs are commonly represented as "perfect for families," "loving companions," "huggable," etc.
The presence of protection dogs in private households has been criticized, however, as entirely too risky.
"[The kennels that sell protection dogs to the public] are in fact breeding their dogs for edginess, poor impulse control, a lowered bite threshold, and in some cases an inborn 'jump in high and bite' motor pattern. Many of these dogs display what we could best describe as ADHD-like behavior – difficulty with focusing and easily distracted, easily wound up, unable to self-dampen once excited. These are abnormal and sometimes dangerous traits in any dog, but they are certainly dangerous in dogs who are going to be trained for attack purposes. Worse yet if these impulsive, trained dogs are placed in family homes." (Alexandra Semyonova, The Tragic Fantasy That a Protection Dog Can Make a Reliable Family Pet.)
On October 24, 2014, 7 year-old Logan Meyer of Hustisford, Wisconsin, was mauled to death by his parents' Rottweiler. They had been training the animal to be a protection dog. The case resembles the killing of a boy in 1997 which led to second-degree murder charges. A woman named Sabine Davidson owned two Rottweilers that she was training in Schutzhund (German for "protection dog"), which includes protection dog training as well as other types of training. Her Rottweilers killed an 11-year-old boy as he was waiting for the school bus. She was convicted of second-degree murder. (Read the case at http://caselaw.findlaw.com/ks-supreme-court/1459039.html.)
Others killed by dogs intended specifically for protection include Kayla Marie Lee (5 years old killed by Rottweiler "on loan" for protection in Northglenn, CO), Sasha Brown, 4 month old killed by Rottweiler purchased for protection in Chicago, IL), and Louise Cooper Gantt (65 year old killed by own Rottweiler for protection in Richland County, SC).
At least one protection dog was a pit bull that was trained as a protection dog AND as a service dog. Owned by a wheelchair-bound, handicapped woman, the dog had to be euthanized after it attacked a third party who did not provoke it in any manner. (Dog Bite Law, Can a Service Dog Also Be a Protection Dog?.)
Police dogs have been known to misread the behavior of humans and inflict severe injuries without provocation. Attorney Kenneth M. Phillips represented a tourist who was invited into a police car but attacked by the police dog, incurring injuries that resulted in a 5-figure settlement against the police department. A "retired" police dog inflicted such severe wounds on the officer's 4-year-old son that his leg had to be amputated. (NECN.com, 4-Year-Old Loses Leg In Attack By Officer Dad's K-9.)
Injuries inflicted by a dog that is trained to attack can result in a conviction for second-degree murder or other felonies such as that set forth in California's Penal Code section 399. It also will result in civil liability in every American state as well as other countries that follow English common law. (See generally, Dog Bite Law, Plain English Overview of Dog Bite Law.)
The use of protection dogs to do double-duty as guard dogs and family companions therefore is fraught with risk.