While I was taught that Roman mythology was essentially Greek religion with a fresh coat of paint, when reading about the rituals and beliefs of the Romans it looks more like they were different pantheons with some external differences. I know that after the Romans incorporated Greece they began to copy many of their cultural practices, and they began to link their gods to the Greek deities. But how did the religions get so similar in the first place? Did the early Romans just have similar gods, or did they make major changes in the late republic to their pantheon?
Edit: For example, Ares and Mars seem like the same god, both male war gods who are sons of Hera and Juno respectively. But while the Greeks had disdain for Ares and though of him as a crude, bloodthirsty and arrogant god, the Romans considered Mars to be a paternal figure, who protected the home, had great honor, and even had some connections to agriculture.
There were many greek colonies on the italian mainland long before there was a Roman Empire, Neapolis (Naples) Syracuse, Tarentum (Tarento), Rhegion (Reggio), Croton ( Crotone) being some examples. Southern Italy was often referred to by even the Romans as Magna Greaca. The Greek pantheon would have been familiar before the conquest of Greece. The differences in how the Romans and Greeks saw essentially the same gods can be explained quite simply. The nature of the Gods changed over time and differed from place to place even in the greek world. A mycenean greek would have had a very different idea of say Apollo than a later Hellenistic period greek. In Homers time Apollo wasn't associated with the sun, yet by hellenistic greek times he was. It seems natural then that if changes in the understanding of the nature of the Gods changed in greece, that there would also be changes over time in how they were seen in Rome.