In English, we have two articles - "the" is the definite article and "a/an" is the indefinite article. Latin, on the other hand, lacks articles, definite or indefinite. Indeed, the great first century Roman rhetorician Marcus Fabius Quintilianus observed that Latin did not need articles ("noster sermo articulos non desiderat . . ."). I bring this up because one of my partners recently sent me Bryan Garner's Usage Tip of the Day observing that "Traditionally, 'Magna Carta' did not take a definite article: one said 'Magna Carta,' not 'the Magna Carta.'"
In recent posts translating bits of Magna Carta, I was decidedly catholic in my usage (see, e.g., It’s Magna Carta Friday!). I suppose that the traditional omission of "the" is truer to the original Latin, which as I observed above does not use, or have need of, articles. However, "Magna Carta" in English means "great charter". The English meaning inclines me to want to add "the".
The U.S. Supreme Court also appears to be undecided about whether to use or not to use "the", as illustrated in this opinion penned by Justice Kennedy:
The right to petition traces its origins to Magna Carta, which confirmed the right of barons to petition the King. W. McKechnie, Magna Carta: A Commentary on the Great Charter of King John 467 (rev.2d ed.1958). The Magna Carta itself was King John's answer to a petition from the barons. Id., at 30-38.
Borough of Duryea, Penn. v. Guarnieri, 131 S.Ct. 2488, 2499 (2011).
I've notice contrasting approaches to the use of the definite article in other contexts as well. A British speaker of English, for example, might say that "He's in hospital" whereas an American would likely say "He's in the hospital". H.W. Fowler points out that older works of literature tend to acquire "the" as in The Odyssey, but more recent works don't. James Joyce's Ulysses is just Ulysses not The Ulysses. The same could be said for Alfred, Lord Tennyson's Ulysses.