When was a pair of Greek and Roman gods first identified?

Question

There is a canonical correspondence between some Greek and Roman gods and goddesses, for example Ares and Mars. However, these two were originally different deities and quite different in nature. My question concerns pairs of Greek and Roman gods like this, which were originally different but were later identified. This excludes Apollo(n), as this god seems to have been always the same, not identified later between the two mythologies.

What is the first known (written or other) identification of a Greek and Roman god? How does the identification manifest itself? Is it explicit ("We call it Iuppiter, they call it Zeus"), is it a Greek story retold with Roman names, or something else? An unattested identification is also fine if it is referenced by a later source.

I took a look at the questions about differences between the pantheons and the transfer of the Greek mythology to Rome, but they do not seem to address my question.

I asked about the same thing half a year ago on the Latin site but there have been no responses yet.

Answer 1

For the sake of future readers, here's some info taken from my answer to this Latin.SE question; I'm not at all sure I've found the first syncretism, and interpretatio graeca definitely happened earlier too (since Herodotus is quick to connect Greek and Egyptian gods), but this is the oldest written source I've found that concerns specifically Roman deities: it predates Tacitus's Germania by about a century.

In Dionysius of Halicarnassus's Roman Antiquities 2.34 (first century BCE):

Μετὰ δὲ τὴν πομπήν τε καὶ θυσίαν νεὼν κατασκευάσας ὁ Ῥωμύλος ἐπὶ τῆς κορυφῆς τοῦ Καπιτωλίου Διός, ὃν ἐπικαλοῦσι Ῥωμαῖοι Φερέτριον, οὐ μέγαν (ἔτι γὰρ αὐτοῦ σώζεται τὸ ἀρχαῖον ἴχνος ἐλάττονας ἢ πέντε ποδῶν καὶ δέκα τὰς μείζους πλευρὰς ἔχον)

Then, after that triumph, and the sacrifice, Romulus built a new temple to Zeus (whom the Romans call "Feretrius") at the top of the Capitoline Hill. It wasn't very big—the old foundations still survive, and the longest side is less than fifteen feet long.

(Translation and emphasis mine.)

This was the Temple of Jupiter Feretrius, the oldest temple in Rome, where the spolia opima were brought after a war. According to Nepos, it fell into ruin sometime before Caesar and was rebuilt by Augustus.

Answer 2

You got a lot of questions. I will try be shorty and focus on When was a pair of Greek and Roman gods first identified? In advance there is no objective answer for that. If there is I will be glad to know.

One of modern Writers on this subject, see Aarom P. Jhonson's Work, believes that Interpratio Graeca was more than a phrase, such as an act in itself, which marked a procces of a Greek cultural a and intelectual conquest of native knowledges. I recognize the use of the phrase "interpretatio romana" by Tacitus, however seems that you are looking for the correpondence of this phrase in Greek-Roman history beside the correspondence of the gods. It looks like in some point there was someone that said "this is the mapping or correspondence of the gods from Egypt to Grece" for example, but there is no certain registry about that, and you seem to be looking for whom that said "this is the mapping or correspondence of the gods from Greek myth to Roman myth", right?

So, we gotta observe that first indentification of some interpretatio between mythologies was made by Tacitus, or registred as if. So he was the first known to explicitly talk about mapping or the "interpretatio" , he was the first the made the link, as the other only talked about it without saying nothing about more than one culture explicitly. Hope it helps!