“Effectively, what he’s saying is that even though there’s a guarantee of birthright citizenship,” explains Evan Bernick, an assistant professor of law at Northern Illinois University and co-author of The Original Meaning of the Fourteenth Amendment: Its Letter and Spirit, “the president can kind of turn it off by declaring an invasion and try to remove whoever he says is invading…It’s not even a loophole, it swallows the entire guarantee.” The fact that Trump referred to a foreign invasion in his campaign video, he adds, suggests they might be anticipating litigation and trying to “boost as much as possible their very minimal odds.”
This is an extraordinarily dangerous approach. He'll basically be declaring them to be enemy combatants. That means he can involve the military. That means he can declare protesters to be giving "aid and comfort" to enemies, and charge them with treason.