"In the event Israel is militarily defeated, and is forced to to accept a humiliating and devastating peace, or worse, is invaded and occupied, perhaps even dismantled as a country, what will happen?"

I'm having a hard time seeing how this could happen. I think Iraq would need to fail first- and that would of course be worst than Israel in terms global impact. Of course it's all a matter of degree, you could say this has already happenned in regards to Israel- and if the situation in Israel remains about the same as it has for last few decades into the next few decades it's pretty grim situation.

"In even the least horrific of these scenarios, there would be a massive emigration from that country. There are approximately 3 or 4 million Jewish Israelis, and with no adjoining country willing to take them, they will have to travel, or be evacuated by air and sea."

I think that these 3 or 4 million Jews would rather fight to the death than go elsewhere. And faced with kind of stark prospect, the Jews will simply win any war they would be faced with. And on top of all this, Israel is US ally. So you are facing a very unlikely outcome. If you were to kill half of Jews with nukes or WMD, then you going to have a nuclear war with tens to hundreds of million of people dying which again makes whatever is going on in Israel fairly irrevelent.

The simply truth is that the terrorist can't win. But of course it depends upon what you define as winning. There is no doubt you could say the terrorists have already won by starting a war with a superpower, and it's possible they could even succeed into turning this into a Total War- though if we have ANY sense we could easily not get to this situation.
If the condition that defines America winning is no war anywhere in the world and everyone in some blissful paradise, we can't win that war. But it's fair to say that if the world gets into a Total war, that America has lost- lots of energy and effort has been expended in avoiding this, and humans everywhere are going suffer dire consequences if this happens.

The terrorists aren't even going to win this war as much as could be said that Saddam won the Gulf war I. In other words you might think [because you are hopelessly deluded] Saddam won Gulf war I -because he survived the war. The terrorist are not going to be this successful, but they *have* already inficted a large amount pain and death and might be able to inflict more misery- perhaps significantly more misery than they have to date accomplished.
The terrorists can no more win this war that they had the capablity of defending Saudi Arabia from Saddam invading the country in gulf war I. Which is about as absurd as having house fire be your baby sitter.

No terrorism or terrorists has ever won a war. Terrorism may have been part of wars that have been won. And of course murderous thugs who operate using terrorist tactics can become leaders of countries- but these countries don't prosper, they endure as a person loaded with parasites endures. The only thing terrorists are winning is a period time in they live in deluded fantasy. And their activity in the best possible light is about a holy as goat fucking.

"The Europeans want no part of them, they've been anti-Semitic for centuries. You know the only place in the world which will be able, or willing to have them."

No worries the Jews of Israel wouldn't want any part of Europe- why would anyone want to live in European slums?