Why would countries like Iran, Cuba, Venezuela, Mexico
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If that ever happened they would feel the wrath of our military. Do you really think our military would stand for that? NO WAY
They wouldn't come bearing uniforms of their country, they would be dropped off at the border, dressed like civilians and join the festivities. There are so many illegals here who's to say where they came from?
When fighting stops ... leave.
I could easily see certain Middle Eastern countries shipping their most hardcore American hating radicals here. Hell, they are fighting us in Iraq and Afganistan, why not bring the fight here?
Maybe a bit far-fetched, but it's really not that implausible when you think about it.