This depends on how you define the title of "guard dog".
What is the difference between a true guard dog and a family guard dog, or watchdog as commonly referred?
Many people misinterpret the term "guard dog". A guard dog guards something, be it a commercial office property, a locked home filled with valuables, or other area that is quarantined and secure to casual entry.
A guard dog is trained to attack anything, or any one that encroaches, trespasses, or steps foot on its territory. Unfortunately, this could be an innocent person, child, or other animal that was never a threat!
Guard dogs, also referred to as patrol or sentry dogs have a low level of socialization and are typically chosen from a more naturally aggressive breed than Great Danes. Once put "on duty", they do one thing, guard and protect an area and anything that encroaches upon it. There are no commands, or handler/trainer control, the dogs are on their own and programmed to attack!
True guard dogs are very aggressive, territorial and defensive by nature. With limited socialization and obedience, they react on instinct. Typically only a handler and select individuals can control a guard dog. When off duty, they are not good around children, other dogs or family members.
We can see why a Great Dane as a guard dog, in the true meaning of the term, is not a suitable candidate. Centuries of select breeding has refined the modern day Dane into a companion animal. Breed standard of the Great Dane states character and temperament as "always friendly and dependable", this is a far cry from what makes a true guard dog.
Our opinion is that a Great Dane is perfect as a watchdog, family guard dog if you prefer, with some true guard dog qualities.
Watchdogs encompass a wide variety of breeds ranging from the smallest toy breed like a Chihuahua, or Miniature Pincher, to giant impressive breeds like a Great Dane. Watchdogs have a common similarity, they will bark, yip, grumble or shout when something is out of the ordinary.
Employing a Dane as a family guard dog for home security is a plus. The boisterous, mighty blast (bark) will send most intruders fleeing for their lives, a low rumble and a statuesque gaze through a bedroom window will cause a would be intruder to freeze and turn to stone. This in a sense is guarding as we see it.
In summary, a properly raised and socialized Dane is not a liability, aggressive behavior and unexpected attacks will not be a problem. The trustworthy Dane will thrive in a family environment and is quite gentle with small children and other animals.
Their timidness and suspicion towards strangers is commonly addressed with passive confidence. Their sheer size and powerful deep bark is very intimidating and a wonderful deterrent towards a would be attacker.
A Great Dane possesses awesome power and strength and is more than capable of taking down an intruder at will.
Certainly Great Danes can be trained as guard dogs, or personal protection dogs, but by nature, they thrive and excel as watchdogs, holding calmly and silently their power and force .
Better yet, a Danes sheer size, piercing gaze and intimidating apperance in most cases will thwart any chance you'll be in danger before the situation arises.
Should a bold intruder ignore the warnings, a Great Dane is aptly equipped to defend its territory with the speed and ferocity of a mighty boar hunter.