To the question
Are there any Christian traditions that speak of single nations being under the protection of an angel?
The answer is yes. There is the common view among Church Fathers, consistent with Jewish Rabbis, that angels are entrusted to the spiritual care of kingdoms/nations.
This article NATIONS AND LANGUAGES, THE SEVENTY: | Jewish Encyclopedia says:
According to the Rabbis, each of the seventy nations is placed under
the protection of a special angel, except Israel, whose protector is
God Himself (Gen. R. xxxvii.).
And this article Angels | New Advent says:
Such appearances of angels generally last only so long as the delivery
of their message requires, but frequently their mission is prolonged,
and they are represented as the constituted guardians of the nations
at some particular crisis, e.g. during the Exodus (Exodus 14:19;
Baruch 6:6). Similarly it is the common view of the Fathers that by
"the prince of the Kingdom of the Persians" (Daniel 10:13-21) we are
to understand the angel to whom was entrusted the spiritual care of
that kingdom, and we may perhaps see in the "man of Macedonia" who
appeared to St. Paul at Troas, the guardian angel of that country
(Acts 16:9). The Septuagint (Deuteronomy 32:8), has preserved for us a
fragment of information on this head, though it is difficult to gauge
its exact meaning: "When the Most High divided the nations, when He
scattered the children of Adam, He established the bounds of the
nations according to the number of the angels of God".
I do not know when the guardian angels of the nations got their commission to explain some fallen angels being princes of certain nations.
The Jerusalem Bible Popular Edition note on Dan 10:13 has:
Michael ('Who-is-like-God?') is the guardian angel of the people of