Sooner or later a terrorist organization will obtain and deploy an effective weapon of mass destruction.
It is most likely that the WMD will be a chemical or biological weapon. Less likely would be a nuclear weapon. If it is a nuclear weapon, it is likely that the terrorist organization will not have created the nuke on its own. Most likely, any nuclear weapon would be from a country that already has nuclear capability, although it is possible that only the weapons grade material will be obtained and the terrorist organization will engineer the weapon on its own. So it is most likely that one of the known nuclear powers will have either willingly or not, supplied the enemy with the nuke(s).
The known nuclear powers are:
8) North Korea
South Africa gave up its nuclear weapons. This has been confirmed by multiple sources.
Israel has nuclear weapons. They are unconfirmed and unofficial, but they do posses them. It is estimated that they posses about 25 nuclear weapons.
I do not see Russia as using WMD except in defense. The old Soviet Union turned their ships around to avoid a potential nuclear confrontation. China is NOT going to attack anyone with nuclear weapons (even Taiwan), except in defense. I believe that England and France would fall into this category as well. Israel is not going to start throwing nukes around unless they are being militarily defeated. I find it very unlikely that India would start anything either, except in defense. This leaves out North Korea and Pakistan. I do not see them using WMD, although it would not surprise me if they helped a 3rd party obtain nuclear capability. Any nuclear attack could originate from Iran as well once they have completed enough development. The weapons themselves could be from any of the countries that have them. Even though the weapon(s) may have been from any of these countries, it is more than likely that the executors of the attack would be from an Islamic country. The key is that the ideology of the organization that launches the attack(s) will be based upon Islam.
The worst reaction is where the U.S. does not hold any governments responsible for the initial attack. The attack itself would likely be somewhere in Europe or Israel. If we did not respond forcefully, the West would probably not be united for very long. They would begin to disagree with each other along with us. If the enemy were not knocked off balance enough, the enemy would continue to build new capabilities. It is likely that the next attack that would follow would not just be one or two cities. I would expect the U.S. to be the primary target at that time. The most important part is the lack of response by the U.S. after the initial ‘crisis’. Unless the counterattack was effective, the enemy would continue to gain capabilities and strength. As the support for the concept of repeated suicide attack demonstrates, our enemy is very likely to use any WMD if they get their hands on them. In other words, threat of retaliation will not be effective.
A strategic defensive posture works when both sides have an incentive NOT to fight each other. If either party lacks this important restraint combined with the desire to engage in warfare, armed conflict actually becomes impossible to avoid. If the conflict does not have enough attrition to drain the resources to a level where it cannot grow, it will build up until it explodes. It has been 70 years since the last Great War. History has shown that human nature requires these conflicts every so often. Maybe we can break this pattern, but I doubt it. The volatile situation we have with Islam is due to the fact that so many issues present are of the type that are worth engaging in warfare to defend or eliminate. This is part of the reason for the rampant irregular warfare that is present throughout the Islamic world and also demonstrates that the conflict is widespread. Repeated suicide attack is occurring for only the 2nd time of all of recorded history and is also present throughout the Islamic world. All of this adds up to my belief that if we do not initiate war against these issues, then the war will surely come to us.