The quote says “if war actually be levied, that is…”

So you see the phrase “that is”? In the English language, we understand that to mean defining the preceding term. The words following “that is” are therefore defining what it means to levy war in this context.

And it’s easier to find interpretations of the term by modern judges than to dig through English case law, so here’s one: https://www.casemine.com/judgement/us/5914a8e8add7b04934706331