Suppose you found this nice girl to get married to, and you ask her to sign a paper if she ever wants out of this marriage, you will kill her, or lock her in by FORCE!
I think a far more likely scenario than several friendly countries in North America is permanent hostility among them, leading to arms races, cold wars and likely the occasional hot one
Wrong, again. If we had three different countries they would have more in common in culture, religion, history and language. It would be difficult to assume that these three countries would be like us and china, or us, and the USSR. After all, us and Canada get along just fine.((sorry to say that about Canada to the rest of the foaming at mouth on this forum)).
You mean like North and South Korea, China and Taiwan, North and South Vietnam, Iraq and Kuwait, India and Pakistan (except for religion), all of Europe throughout most it's history?