This article describes each of the Human and Creature groups' loyalties and towards which other groups are they hostile or allied. In general, all of the groups are hostile towards Garrett, but a thief must know how to make his enemies fight one another in a subtle and cunning manner.
Humans overall have pretty plain lines of allegiance, usually separating themselves by class or religion.
Hammerites won't engage other humans except thieves and criminals, Garrett included. The exception of Garrett is at the conclusion of Thief: The Dark Project and on the streets of Thief: Deadly Shadows if Garrett sways favor (This is not carried over into Hammerite Missions; they will attack Garrett on sight).
They will engage anything non human, unnatural beast or undead, on sight. The Hammers have an exceptional hate towards Pagans. In Thief: Deadly Shadows, Hammerite Guards help the City Watch fighting enemies. If they encounter Pagans in the streets, combat will break out between them.
The two groups were certainly ideologically against each other, but it has not been confirmed if there was any violence between the two groups. Full scale conflict never came to fruition due to the weakness of the Hammerite order after the death of The Trickster.
Guards and City Watch Officers are generally peaceful towards the average human, but will attack hostile creatures, non-human entities and undead (except Lauryl's Spirit) on sight. They both will attack criminals if observed as intruders (guards and watchmen will always attack thieves in the first two games, but are rarely in contact in the original missions, with the exception of Davison's crew in Shipping... and Receiving). City Watch are always hostile to Garrett. The guards are as long as Garrett is intruding. The only time guards are neutral are the guards outside Bafford's Manor, as long as Garrett obeys their commands to leave or is non hostile. Guards will still attack Garrett in the first two games even if he is seen outside their Lord/Lady's property. These occurrences are few, however. In Deadly Shadows, non-City Watch guards who spot Garrett on the street will not bother Garrett, unless he commits any criminal activity.
The City Watch was ordered to kill Pagans on sight in Thief II: The Metal Age, but by the time of Deadly Shadows, they just consider their presence to be a nuisance.
Pagans won't generally engage humans, except on certain conditions and when on their own territory. The only exceptions make the Hammerites who are enemies with the Pagans. All creatures allied or summoned by Pagans including Chaos Beasts won't attack the Pagans themselves, but will attack any other human or creature not a part of the Pagan culture. During the rise of the Mechanists, them and The City Watch were openly hostile to the Pagans. However, with the fall of Karras and the lax attitude The Watch has with The Pagans, has allowed them to further their influence in The City.
Rich or poor, unarmed peoples will always run to the nearest uniform with a weapon. They will react negatively to monsters, creatures, intruders and other things they deem a threat to their safety. Otherwise they are neutral to other persons and groups. Allegiances vary from person to person, as some are devout followers of the Hammerites, some may be secret pagans, and some do not mind being around criminals.
Creatures and beasts are more chaotic, usually allied with their own species, or in the case of Chaos allies, whomever The Trickster dictates.
Elementals will attack any non-elemental entity. The only exception goes for magically summoned elementals which won't fight their creator, or his allies. Fire Elementals may be seen allied with Chaos Beasts.
Air elementals are only seen when casted by Mages. They are short lived and attack anything that is not allied to The Mages.
The Kurshok will attack any intruders, especially Pagans, due to their attempts to retake the Kurshok Crown. Gravid, the imprisoned Kurshok will be pursued by Guards, Watchmen, and Hammers if seen escaping his cell.
The Undead will attack anything alive, sentient or not. Though they are not seen acknowledging each other, they have the known tendency to amass and help one another more than the other groups.
Animals will attack anything not from their own packs or types. In certain missions Wild Beasts can become allied with Chaos Beasts.