Why isn't terrorism an existential threat to the US? Or rather, why don't I believe it is?
Conventional terrorism is not an existential threat to the US because there aren't very many people here who want to be terrorists. Let's list the potential resident nutjobs who might commit such acts:
1) Tim McVeigh-style racist scumwranglers: small in number. Their most terroristically useful members are probably in Iraq or Afghanistan right now.
2) Arabs/Pakistanis/Chechens affiliated with Al-Qaeda: also small in number. The Chechens tend to be focused on Chechnya. Most of our Arab citizens are Christians, who are not fond (she said, with elegant understatement) of the Osamamaniacs; most of our immigrants from the subcontinent are not Pakistani.
3) Converted muslims: even smaller in number. Hard for them to win the trust of Al-Qaeda and make it into a cell.
4) Crazed suicide cultists: even tinier. Twelve people do not pose an existential threat to the United States unless they gain control of the Blossom syndication rights.
Moreover, in any immigrant group, the number of people even theoretically prone to carrying out terrorist acts, rather than tacitly agreeing with them, is tiny. Israel has millions of angry Arabs on its border with nothing to do but seethe as their economy falls into ruin. Even during the height of the infitada, pre-wall, you're talking about a handful of aimless young men.
Terrorism is not, as many have pointed out, a product of poverty; it is a product of lack of opportunity + limited political rights. Immigrants here have full political rights and plenty of economic opportunity. The United States does not, as France does, ban their hijabs; nor deny them citizenship like "guest workers" in any number of countries; nor deny them employment like Scandinavia; nor does it (AFAIK) wink, as Britain does, at practices like fourteen-year-old cousin marriage that prevent assimilation. We just don't seem to have a critical mass of disaffected immigrants with extreme religious views. It's telling that 9/11 relied on visitors, while bombings in Europe and Bali tend to employ local talent.
It's also harder for Al-Qaeda to get here. North Africa is very close to Europe. Most Arabs or Pakistanis trying to make their way up through Mexico would attract quite a bit of notice before they got there.
But what about nuclear terrorism?
Well, a bigger worry. But prime targets like New York and DC are, or so I am given to understand, now fairly well covered with radiation detectors. The kind of primitive bomb that terrorists are likely to be able to get their hands on is low yield; they aren't going to be able to wipe out a city with a suburban ground burst, and they'll have a hell of a time getting such a bomb into the air.
Also, the countries they might get bombs from--North Korea, Pakistan, possibly Iran in the future--don't have that many of them. How likely are they to give a precious nuke to a group they don't control? It's not inconceivable. But it's not very likely either.
At this late date (oh famous last words) I think it's unlikely that the infamous missing Soviet suitcase nukes exist. First of all, these bombs require expert maintenance; you're now talking about an armament that is 20 years old. Second of all, if someone had one, and had sold it to a terrorist group, my guess is that it would have gone off somewhere by now.
But even if there were a nuclear blast, it would be an unimaginable tragedy, but not an existential threat. The Soviet Union was an existential threat to the United States. Even 200,000 dead in a horrific tragedy is not.
Obviously that is an unacceptable cost that we should take steps to avoid. But I consider the probably of such an outcome extremely low, at least in the next ten years or so. I very much doubt that Al Qaeda or its brethren will manage to achieve anything as big as 9/11 again in the foreseeable future. Not because Homeland Security is protecting us, but because anyone who tries to hijack a plane will face a mob of passengers who believe they are already dead. Even a machine gun probably wouldn't be an adequate weapon in that case, and the difficulties of getting a machine gun onto a plane, then deploying it without explosively decompressing the plane, are very large.
Udpate A friend points out that the line about racist crazies being in Iraq and Afghanistan could easily be taken the wrong way. I was not in any way trying to imply that the military is full of Aryan Nation types; only that such groups probably can't stage an attack without members who are ex-military, like McVeigh himself. Those members have probably been called back to the Guard and sent to Iraq. Or so I mote. I do not think that these are a sizeable portion of the military; only that the military, like any group that contains millions, is likely to have a few rotten apples scattered in there.