"You've heard of warfare, right; it's when multiple countries fight for some cause."
The first sentence could be seen as more of a rhetorical question than actual. This is a question of style, and style guides and personal preferences may vary. See here, where it is argued that:
Rhetorical questions can be ended with either a question mark, an exclamation mark or a period. Using a question mark is probably the most common choice, but it is really up to the writer to use whatever punctuation matches best the intent of the rhetorical question.
I am sure that if they can end with a period, they can also end with a semicolon.
Since it is obvious that the listener will have heard of the warfare, the "right" here is not very different from asides such as "shall we say", which are explicitly allowed to lack question marks under CGEL's guidelines (Cambridge Grammar of the English Language, p1733).
In general, of course, a question doesn't end with a semicolon (unless one is writing in Greek or Church Slavonic).