Zeus, the Skyfather, Lord of Lightning and Thunder, is the King of the Gods of Olympus according to most treatises on Classical Mythology. Most treatises on Classical Mythology, however, ignore the long history and historicity of the cults of the gods of Olympus, where they came from, and what happened to them. Consequently, these details are shrouded in over-simplified myths and metaphor.
In reality, Zeus came upon the Greek pantheon as a new god prior to its identification with Mount Olympus. At the time, a loose collection of gods including Artemis, Athena, Dionysos, and the Mistresses (Demeter and Persephone), Hephaestus, Hermes and Hera, along with a number of gods that later became more or less forgotten—Paean, Erinya, Eileithyia, and Despoina ruled a small pocket dimension that was tied to Greece on planet earth, allowing them to occasionally roam over this circumscribed geographical area. Over this loose collection of gods, Poseidon was champion and “first among equals”—a powerful force individually, but their loose collection made them vulnerable to another militaristic pantheon, the so-called Titans of Mount Othrys, who’s own pocket dimension was anchored in another part of the Greek peninsula, putting them at odds because of proximity.
Zeus arrived with some of his young and untested allies: Ares, Apollo, Hades, Aphrodite, Herakles, and others, and due to his own war-like nature and off-the-charts personal magnetism and physical power, was able to unite the disparate Mycenaean pantheon into a powerful force. They set up their paradise on Mount Olympus (in reality, a pocket dimension called Olympus, which is most often accessed via a semi-permanent portal on Mount Olympus in Greece) and waged the terrible war known as the Titanomachy. This was one of the first and most terrible examples of open warfare between pantheons of Outsiders, and the triumvirate of Zeus, his “brother” Hades, and Poseidon, the champion of the region before the arrival of Zeus and his entourage, proved to be unstoppable.
The Olympian pantheon later made war against two smaller pantheons; those later known as the Etruscan gods and the Roman gods. After suffering a number of losses in lightning raids that left their forces imprisoned and depleted, the Etruscan and Roman pantheons merged, and brought a titanic battle to the Olympians. Echoes of this struggle remain in mythology in garbled form as the Gigantomachy, but the effects of the war are better illustrated on the practices of the worshippers of these pantheons. The Etruscan pantheon, as transmitted to archeologists, adopted more and more Roman and Greek influences, and the Roman pantheon adopted the stories and personalities of the Olympians, although in most cases the names remained Roman. As with the titans, many of the original Roman and Etruscan deities were imprisoned and their places taken by Olympians, who adopted their names and titles for local worshippers, while others were integrated into the Olympian pantheon.
There were some bitter and brutal moves during this integration. Ares was slain and his reborn soul was hidden by the Romans, for example. Later, the Roman Mars took his place in Olympus, leaving Ares as an exile with no knowledge of his heritage for many centuries. Today, Ares remains a bitter outsider, unconnected with the Olympians, since Zeus and the triumvirate had to “adopt” Mars as part of the cessation of hostilities with the remaining Roman pantheon. Hades “adopted” a number of deities with similar interests—Orcus, Ploutus, etc. and announced that they were permanently part of his entourage.
This was the last great Outsider war in which the Olympians participated—following the naked aggression of Zeus and his brothers against the Roman and Etruscan pantheons, few other outsiders would trust or deal with the Olympians anymore, and many would rather make common cause with their rivals against the Olympians rather than give the powerful Zeus a chance to work his way into another pantheonic conquest. That said, two additional powerful deities were later associated with the Olympians during the later Roman times—Isis, a goddess from the Eqyptian pantheon who dallied for many years with Poseidon and had a powerful alliance with him (and him alone, not with the rest of the Olympians). Zeus also befriended Mithras, a wandering deity who was part of an pre-Avestan pantheon that imploded in brutal warfare and politics, and the Mithraic mysteries were an important part of Olympian worship for some time. Mithras eventually wandered again, although his friendship with Zeus remains an important trump card that Zeus can pull when needed, for Mithras was a solar figure and infamously powerful warrior.
The Olympians had a long-running “Cold War” with a Celtic pantheon that was separate from the later attested Irish and Welsh mythologies, but this never erupted into full-scale war. While historically the Roman mortals did eventually conquer and assimilate most of the continental and many of the insular celtic groups, their pantheon was able to form a temporary alliance with the up and coming Teutonic pantheon, an early incarnation of what is later known better to us as the Norse pantheon, where it was actually transmitted to archeologists via writing. Having just come off a war of their own, in which the Æsir and Vanir gods merged to form a much stronger Teutonic pantheon, they were a militarily powerful group and rival for the Olympians that the Olympians were not ready to tackle. Some small part of the warfare and politics between the Teutonic—later Asgardian, Celtic—later, Avalonian, and Olympian pantheons is reflected in the complex relationships between the Celtic, Germanic, and Romano-Greek peoples, but as all three populations were eventually Christianized and became significantly intermingled culturally and ethnically over time, later events in the pantheons are not reflected in mythology or history, but in the secret history of the Outsiders, which is unknown to most peoples.
This later history, however, mostly coincides with that of the other pantheons—as worship of the Outsiders as gods has faded and failed, they have found that their connections to Earth have become less stable and predictable. This has also led to a dampening of hostilities between pantheons, as without easy access to Earth and “the Porch”, they don’t have the means to make war on each other. It also means that individuals are often stranded on Earth, and sometimes are even killed here. Re-embodied without the ability of their “home” pantheons to raise them to take the same place that they held before their deaths, many pantheons have holes missing in them, and the presence of Exile outsiders, who have little or no knowledge of who or what they are, is growing. Some believe, for example, that Samson or Beowulf (or perhaps both!) were actually lives of Herakles, who while wandering far from Olympus was slain, grew up amongst a foreign people when re-embodied and reborn, and became a local hero to them before being killed again.