So I was wondering about why the Greeks and the Roman's have very similar mythologies. Essentially I'm wondering why they're similar but also why they're different? Like was there some sort of a schism at some point? Is it pure coincidence?
Side question, a history buff friend of mine once told me that when the Romans invaded Greece, they did not raze their temples as they felt that the Greeks worshiped the same gods they did, they just called them by the "wrong" names. Apparently they effectively were like "silly Greeks, this is Jupiter, not Zeus". Is there any truth to that?
There are some really broad things to cover in here because Greek and Roman religion intersect at many points, including thousands of years before there was such a thing as either Greece or Rome.
The first point of contact takes us all the way back to c. 3000 BCE with the Proto-Indo-European peoples. Proto-Indo-European (PIE) refers to the culture that spoke the language that split and developed over time into most of the languages of modern and historical Europe, Central Asia, Iran, and northern India. By tracing which words shared between recorded languages share common roots, historical linguists have traced this culture to the Pontic-Caspian Steppe around 3300-2300 BCE. That means they were probably the same people identified by archaeologists as the Yamnaya culture. With this information in mind, it's not just language that we can examine. Many of the cultures within that larger Indo-European family share similar gods and mythologies that can also be traced back to that common root.
These include things like sun and moon deities, a pair of divine twins associated with horses, personified spirits of bodies of water and the wind, three crones who control the fates of everyone else, a god of metal work, a Mother Earth figure, and a "sky father" at the head of the pantheon.
The sky father is probably the most famous and universal example. Jupiter and Zeus are the relevant examples and both can be traced back to the Indo-European name "Dyeus Pater." Literally something like "sky father" or "god father." Jupiter is just the two parts of that name pushed together, and Old Latin texts even called him Dovis, which later became Iovis/Jovis (the root of an expression like "By Jove!"). Zeus is a later form of "Dyeus." In Ancient Greek, different declensions of Zeus were actual written as "Dios." Zeus was often called "Zeus Pater" (Father Zeus), especially in earlier Homeric Greek.
The Indo-European peoples went through a series of migrations. Several of these pushed people into Europe where one group entered into Greece and their language and traditions gradually evolved into something recognizably Greek. Another, entered into Italy, where they developed into various Italic traditions, including the Romans.
Now that was all sometime before 2500 BCE, so there was plenty of time in which the Italic/Roman tradition and the Greek tradition evolved on their own, but beginning around 800-700 BCE they began to interact. Ancient Greek culture was not isolated to modern Greece. There were Greek city-states all along the coasts of Europe, Anatolia, and Africa from modern Crimea to Spain. Many of these colony cities were concentrated in southern Italy and eastern Sicily, forming a region known as Magna Graecia (Greater Greece). Some of these cities were founded around the same time as the legendary founding of Rome, so the Romans developed in a world where they were in regular contact with Greek culture. There was never any "discovering Greek mythology."
That said, the Romans weren't in Magna Graecia, and they weren't Greek. They had their own traditions. Some developed gradually out of the Italic branch of the Indo-European milieu and some were adapted from the non-Indo-European Etruscans. Prominent examples that they didn't share with the Greeks at all include Janus - god of duality, doorways, choices, and transitions - and Quirinus a war god that may have originated as part of Janus and developed into his own cult. Qurinus was one of the most prominent gods before there was a lot of Greek influence.
Other gods were identified with clear Greek parallels. Jupiter and Zeus, Juno and Hera, Ceres and Demeter, etc. were all very similar. Other Greek gods, like Apollo, were adopted into the Roman pantheon wholesale with almost no changes. On the other hand, some gods were identified as the same but treated very differently by the Romans and Greeks. Mars and Vesta are probably the best examples. Mars is usually identified with the Greek Ares, but in Greece Ares was a god of violent brutal war and the destabilizing or destructive aspects of war. Tactics, strategy, and the finer points of war tended to fall to Athena. Mars on the other hand was more of a god of generals. He was the divine warrior, and represented war as a means to obtain security.
Over time, Greek culture became very influential. The Greeks developed the very effective hoplite phalanx system of warfare, produced excellent crafts, controlled trade in the eastern Mediterranean, and produced literature on a grand scale. All of this permeated into Roman culture, and the early Romans began to identify their gods with the gods in the art and stories of the Greeks. This is called synchronsim and there was absolutely nothing strange about it in the ancient world.
In a polytheistic religion there is less of a barrier to entry for new gods. You've already accepted that there are many, many gods and thus it is not hard to accept that there are still more. At the same time, when foreign gods were very similar and filled similar roles to your won gods, ancient people tried to identify them as the same gods named in another language. Sometimes the explanation was even that the gods went to other countries to play different roles for their own amusement. You can see this in most cultures of the ancient world. The Romans identified their own Venus, with the Greek Aphrodite, who was in turn compared to the Phoenician Astarte, who in her own turn was compared to the Mesopotamian Ishtar, who was further compared to the Persian Anahita.
So it wasn't that they thought the Greeks had made a mistake in calling Jupiter "Zeus," but just an understanding or belief that Jupiter went by that name in Greece. When Alexander the Great went to Egypt, he identified Zeus with the chief god of their pantheon at the time, Amun. When the Persians invaded Greece, Herodotus used "Zeus" to refer to their chief deity, Ahura Mazda. It was just a standard part of ancient Mediterranean culture to identify which gods were similar and which ones were unique between each pantheon. The weird ones were religions like Judaism who refused to accept the existence of their neighbors' gods.
Finally, all of this was happening before the Romans conquered Greece. Frankly, it was already happening before Rome had conquered all of their own region of Italy. Greek trade ensured that Greek culture and Greek learning was a dominant force in the Mediterranean for centuries before the Romans expanded beyond the Italian Peninsula. That said, the level of synchronism and the similarities between the two pantheons increased significantly as the Romans brought more Greek territory into their empire. First they took the cities of Magna Graecia, then the Greek cities on Sicily. Then they began intervening in the wars of the Greek kingdoms in mainland Greece and the Near East. Over a couple of centuries, the Romans absorbed more Greek territory, which led to more direct exposure to Greek culture. This was especially true for Greek literature circulating among and targeting Roman audience, and in the realm of Roman education. Each defeated Greek army led to an influx of Greek slaves, and enslaved Greeks were sought after as tutors for the children of the Roman elite, further exposing generations of Roman Republican and early imperial leaders, priests, authors, and politicians to Greek influence.
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