Nah, ISIS isn't likely an existential threat to the US.
But neither was Nazi Germany. We existed either way. But the world was threatened in a manner that demanded our attention.
ISIS has that potential. Well, the hydra-headed Islamist/Jihadist movement does, ISIS being the most visible current incarnation. And America is certainly threatened by ISIS funded/supported/encouraged attacks. We've seen them already.
Crush them. Be diligant and quick in identifying/responding to the next ISIS. Seek a stable Islamic Middle East, one that reduces future threats, not enhances them.
And after step one, we can't simply tell the Saudi's/Iranians/Yemenis/Shiites/Sunnis/Copts/etc, "Now you guys all play nice together, with the rest of the world, too," leaving it at that.
Unfortunately, we have to be involved. Actually, it isn't unfortunate. When we withdraw, some other (often bad) actor fills the void.
Doc, I really do find the argument that an temporary and limited immigration ban (I have no problem with that word) only strengthens ISIS a very hard argument to buy.
Will they Double-Dirty-Dog Hate us now, while yesterday they only Double-Dog hated us?
Immigration is often a good thing (as my dad knows). Assimilation is too.
Somewhere I have saved an article that speaks of successful immigrant populations to the US "checking their identity at the door." That does not mean that you eliminate every iota of the old-world culture, mind you. But it does mean that you well...assimilate.
Ask the French about the other option.
Middle Eastern immigrant groups have generally assimilated pretty well, minus a Burkha or two and some Sharia Law wild-eyed advocates. This I shall not dispute. But a bit of caution in a violent age isn't a bad thing at all.
We have a 90-Day ban on immigrants from 7 countries. It hardly violates our "core American beliefs" (immigration bans or limits have long been a part of American history, btw......and they are not necessarily xenophobic). 120 Days of no refugees will impact some, but it is a relatively short period of time. I don't get the anger. Really.
No Syrian refugees for the duration? I expect this will be watered down and am certainly fine with that, but it sends a message. That message is not one of hate, but one of controlled borders.
When my dad came to the US from Canada in 1948, he had to show a letter from his bank that he had $1200 in deposit to get the visa to enter. That was 1948. Propose that today and you would be accused of hating all Canadians, or of being a rascist so-and-so, by a significant chunk of the left-leaning voices.
Or just propose a 90-Day immigration stoppage from a whole 7 countries. You get accused of the same thing.
An idea: Two days after taking office, FDR proposed a work-week long "Banking Holiday," to help prevent runs on local banks. It was a brilliant bit of "branding," calling it a "Holiday," rather than an emergency measure (which sounded ominous). FDR knew the power of words. Maybe Trump should have just proposed an Immigration Holiday. :)