one of a race - but which one [closed]

Here's the context of your quotation:

These Articles of War had their origin in a period of the history of Britain when the Puritan Republic had yielded to a monarchy restored; when a hangman Judge Jeffreys sentenced a world's champion like Algernon Sidney to the block; when one of a race by some deemed accursed of God — even a Stuart, was on the throne; and a Stuart, also, was at the head of the Navy, as Lord High Admiral. One, the son of a King beheaded for encroachments upon the rights of his people, and the other, his own brother, afterward a king, James II, who was hurled from the throne for his tyranny.

Here the "race by some deemed accursed of God" (that is, a race which was deemed by some people to be an accursed-of-God race) is the House of Stuart. The two Stuarts in question are Charles II (the one upon the throne) and James II (the Lord High Admiral).

People certainly had strong views on the Stuarts in the 17th century — James I was the target of the Gunpowder Plot (1605), Charles I was beheaded during the Civil War (1649), and James II was deposed by the Glorious Revolution (1688). Even if you didn't think they were a bad race of kings, you would have had to consider them an unfortunate race! So "by some deemed accursed of God" seems to fit, in either sense.

As an American and non-historian I can't definitively say how the British view the Stuarts today, but I can at least quote the popular rhyme from Eleanor and Herbert Farjeon's Kings and Queens (1952):

Four Stuart kings there were, whose names
Were James and Charles and Charles and James.

The first, as history makes plain,
Was ugly, greedy, gross, and vain.

The second, it must be allowed,
Was dense, pernickety, and proud.

The third, if I make no mistake,
Was an incorrigible rake.

The fourth combined, it seems to me,
The vices of the other three.
[...]

However, I think your main confusion here stems from the archaic use of the word "race" to mean no more or less than "lineage." A "race" is simply a family descending from the same root ("Etymologists say no connection with Latin radix") — synonyms in this context include "lineage," "breed," or "family." And just like those words, race can even be used of a purely metaphorical breed, as when Jonathan Swift talks of "the whole race of politicians." The modern meaning of "race" as more or less "skin color" derives from this original sense; the word underwent a rapid period of semantic narrowing in the 20th century.

The other archaic thing going on here, grammatically, is this construction: "One of an accursed race — even a Stuart — was on the throne." This is a rare and archaic use of "even" to clarify the referent of the preceding description. You could omit the word "even" here and it'd still mean the same thing. (As opposed to modern usage, where "even" always has some intensifying or specializing effect on the connotation of the sentence: "I never even killed a bluebottle"; "Mercy even for Pooh-Bah.") I'm sure I've seen this usage before, but I'm having a hard time finding any other example of it.

I have read the context here and think it simply is defining the House of Stuart as 'an accursed race' (from an American perspective) [of monarchs.]. If you see the causes of deaths of the monarchs here almost all of them died tragically, with deaths such as murder, grief, beheading etc. The Stuarts are probably known as the most disastrous English house, and Merville is pointing out that it is 'accursed' that one is on the throne, and also head of the Navy.