Dogs were used widely throughout the Mediterranean world in hunting and warfare. Xenophon's writing in On Horsemanship demonstrates that animal trainers and breeders of the time had a deep understanding of animal psychology. The Romans used dogs to guard their flocks and homes and the modern phrase "BEWARE OF DOG" comes directly from the Latin "Cave canem". There are accounts of slave-catchers who trained their dogs to track fugitives. Dogs were an expected part of any Roman hunting party. Large dogs were also in constant demand in the arena.

Excavations at Vindolanda, Hadrian's Wall, and other permanent bases on the Roman frontier have revealed the skeletons of large dogs. The use of dogs to guard these bases and accompany foot patrols is pretty well-established. Dogs were also used as messengers. The Column of Marcus Aurelius, from the late 2nd century AD, shows large dogs wearing spiked collars and chainmail, although I'm not aware of any source stating that was standard battle dress for Roman military dogs.

There is one account (I think in Polybius?) of Romans attaching firepots to the backs of dogs, who would then run under enemy horses and force them to throw their riders. I don't think we can call this standard practice by any means.

More than likely, the Romans used dogs to guard and herd the cattle and sheep that fed their legions, to patrol their bases, and to assist scouts in rooting out fugitives or enemy pickets. Bringing a dog onto a battlefield, as shown in Gladiator, is extremely dubious.

Sources:

The Western Frontiers of Imperial Rome, Steven K. Drummond and Lynn Harry Nelson.

On Horsemanship, Xenophon.

Policing the Roman Empire, Christopher J. Fuhrmann.

Animals in the Military, John Kistler.

Please, no more sub-par answers. Remember the rules:

If you're choosing to answer a question in /r/AskHistorians, there are three questions you should ask yourself first in turn:

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Certainly.

More specifically, Romans used many breeds, Chief amongst them being the Laconian and Molossus being described by Cato in De Agri Cultura (c.160 BC) as "the hunting-dog suited to chase the beasts of the forest, and the other which is procured as a watch-dog and is of importance to the shepherd"

The Molossus, being bred as a guard dog, was often used in conjunction with patrols or sentries. There is no solid evidence (at least that I am aware of) that dogs were taken directly into battle by the Romans. Indeed, the tight unit manoeuvres and coordinated battlefield preferred by the Roman army almost certainly precludes the use of such a chaotic element in a set battle.

Follow up: what breeds have been most commonly used as war dogs? Has there been one predominant "war breed" in many militaries?

Follow-up question: Has there ever been a military force that did not use dogs in some way? Maybe a civilization that just didn't make use of dogs?

So does this mean dogs were not used in any function similar to Rome: Total War? (A host of attack dogs would be let loose against skirmishers or to break up an enemy formation)