Why not "You are going to defend THEIR country"? "Su" can also mean "their". Or would you have to say "Ustedes van a defender el país de ellos"? If so, why don't you have to use "de ella" when you mean "You are going to defend HER country"? Are some meanings of "su" more readily assumed?
I think "their" should be acceptable, same as "her" or "his", the "su" can work as your (formal, both singular and plural (not here because they say "ustedes", but still), her, his and their. "El país de ellos" sounds awful in Spanish, I have heard it, but I am not even sure that is correct, being honest.
Because any third person singular verb can take either "he," "she," or the formal "you" as its subject, ambiguous meanings are possible when a sentence has a compound subject with more than one gender, has gendered object(s), has nouns with different genders, etc.
In Spanish, the third person plural conjugation of van is understood to be "they go" by default when there are no other contextual clues, and the secondary meaning of "van" is understood to be the formal "you (plural) go." Similarly, the default pronoun of any third person singular verb is "he" when there is no other context. The secondary pronouns possible as subjects of any third person singular verbs are "she" (ella), "it" (unwritten in Spanish but sometimes indicated by the reflexive "se"), and "you" ("usted," formal singular). When you use the genitive case, it is important to remember that either the article "el/los" or the article "la/las" is used before the the object of the prepositon "of." In other words, NO type of possessive (for example, "su" and/or "sus,") is used before the noun when the noun is followed by the genitive case. Also, genitive case prepositional phrases are unnecessary when there IS context.
Ambiguous meaning calls for clarification. In Spanish, this is accomplished by using the genitive case (i.e., putting prepositional phrases after nouns with pronouns that clarify who "owns" said nouns ). These prepositional phrases for third person singular are "de él (of him/his); ""de ella" (of her/her); "de tu" ("of yours/your" singular familiar); and "de usted"(singular formal "you"). When the pronoun that is needed is "its," use "su" because genitive case is unnecessary. For third person plural: use "de ellos" or "de ellas" ("of theirs," masculine gender specific and feminine gender specific) as the default, and use "de ustedes" when you need the plural formal "you."
I'm with you, Babella. "You are going to defend your country" takes a plural subject in Spanish, which is "ustedes," and the "su" has a plural meaning because in English the word "you" has a plural meaning depending on context. In "Ustedes va a defender su país," a plural subject defends a single shared noun "their country." "(All of) you (a plural) are defending one thing, "your paíz." Because "paíz" is singular, it takes the singular number pronoun "su."
In Spanish, the primary third person plural conjugation of van is understood to be "they go" when there are no other contextual clues, and the secondary third person plural meaning of "van" is understood to be the formal "you (plural) go." Similarly, the default pronoun of any third person singular verb is "he" when there is no other context. The secondary pronouns possible as subjects of any third person singular verbs are "she" (ella), "it" (unwritten in Spanish), and "you" ("usted," formal singular).
Protect would be "proteger"--they are similar in meaning but not exactly the same. Also, land is not the same thing as country. I could defend my land as in my house, my neighborhood, my town, or my state--but that's not the same thing as my country. Land would be tierra.
I put "You shall defend your country," which is English future emphatic mood, and is a command in which one person is telling others that they will be compelled to defend their country. It was marked correct.
Any comments from native Spanish speakers? Do you think it should be considered subjunctive mood or imperative/command mood when it is translated back into Spanish? I would love to know your opinions.