Note that you could drop the “the” from the first sentence and still have it make sense.
While we’re at it, anyone want to chime in on these:
a) The interesting thing about the Romans is the series of roads they built.
b) The interesting thing about Romans is the series of roads they built.
A sounds more correct (I guess because “the Romans” refers to the people as a whole), but I tried replacing it with something different and now I’m confused.
a) The interesting thing about the Americans is that they like hamburgers.
b) The interesting thing about Americans is that they like hamburgers.
Sentence (a) is better because “the Romans” is usually interpreted as meaning the people of ancient Rome. “Romans” without the definite article, unless it’s in a specifically classical context, sounds as though it means the inhabitants of Rome today.
a) The interesting thing about the Romans is the series of roads they built. [“The Romans” = people of ancient Romre]
b) The interesting thing about Romans is the way they maniacally weave through traffic on their scooters. [“Romans” = people of Rome today]
For the same reason, in your second example, sentence (b) is better:
a) The interesting thing about the Americans is that they like hamburgers.
b) The interesting thing about Americans is that they like hamburgers.
The style in sentence (a) would sound right if you were referring to Americans in a particular historical context:
In the Revolutionary War, the interesting thing about the Americans was their quasi-guerrilla mode of fighting.
Which of these is preferable depends on context. The first one, which is a simple past tense construction, is probably what you intended. The second one is a past perfect construction, which would be appropriate if you were talking about past attacks on England relative to another situation that is also in the past.
Examples of context for each case:
“The combination of internal strife and geographical vulnerability weakened the nation. For many years, England was attacked by many enemies. The security she eventually attained was dearly bought.” [Simple past.]
“When Harold II came to the throne in January 1066, the fate of his beloved Anglo-Saxon nation hung in the balance. For many years, England had been attacked by many enemies, and now the final crisis approached.” [Past perfect: the past attacks are being considered from the viewpoint of another past event.]
[N.B.: No claims are made for the historical truth of the statements in these examples, I’m just making up stuff to illustrate the grammar.]
It seems to me that in addition to the past/current Romans issue, the use of ‘the’ means that the group as a body, rather than the individuals.
a) The Romans fought the Persians.
b) Romans fought the Persians.
c) Romans fought Persians.
a) means that the Romans collectively fought the Persians collectively - presumably army to army.
b) means that some of the Romans fought the Persian army
c) means that some Romans fought some Persians - there’s nothing to indicate how many, or whether this was official or anything.
I completely agree with Kimstu about the simple past/past perfect (AKA pluperfect) being dependent on usage. If it’s directly related to how “King Pendragon” (somewhat akin to calling the present British monarch “Queen Windsor” by the way) came to be dux bellorum/Paramount King, then the past perfect would be preferable, since it’s talking about events before and leading up to what you’re describing in simple past.
Note that the second choice in the first set would be equally correct with the addition of a smile “a” – the phrase “a long time ago” being commonly used adverbially, the equivalent of “yesterday” or “in days of yore”. (It answers the question “when?”, normally the function of an adverb (then, yesterday) or an adverbial phrase such as this or “in the Fifth Century”.)