could any of the kingdoms at middle earth attack the southern continents?

[Zigûr]

Originally Posted by tom the eldest

Nah ,harad not included in the middle earth category.

For the purposes of this discussion, or in general? Because Harad is definitely part of Middle-earth in the books.

Originally Posted by Morthoron

Did it occur to you that there was simply nothing worthwhile Gondor wanted further south? They got what they required in Umbar and Near Harad, and nothing more interested them. There is nothing to support what you are saying in the text.

Exactly. Given how little we know about Far Harad, it's virtually impossible to speculate. Besides, conquering a little of Near Harad doesn't mean Gondor had the strength to conquer what might have been equivalent to, for all intents and purposes, the whole of Africa, even if they wanted to.
That being said I think the usefulness of elephant cavalry ought not to be overstated as well. As formidable as the Mûmakil were compared to their later descendants, they were not invincible, and elephants make for temperamental and inefficient beasts of war.

Originally Posted by tom the eldest

if gondor can invade to the south,why they not push their advantage and invade the dark lands?

For what purpose? This is treating the situation like a game. It may be fiction, but it also adheres somewhat to the rules of history. Even within the bounds of Tolkien's narrative the Men of Gondor do not appear to have fought wars "for fun" or without reason.