For example: suppose an international terrorist group was to get hold of ten nuclear weapons from a corrupt official highly placed in the Russian military (or from North Korea). Also suppose that they then exploded three of the weapons to demonstrate their capabilities and then demanded some colossal fee to not explode the remaining weapons in seven of the Western world's (unspecified) major cities.
The result would quite probably be a panic evacuation of Western cities with resultant interruption to fuel production (among many other things). If our society's fuel production were disrupted then international commerce would be one of the more immediate casualties. If the interruption to the fuel supply continued for a sufficient time our farmers would not be able to plant or harvest their crops, and catastrophic famine would follow. (Unlike the third world, our agriculture is almost entirely dependent on diesel fuel.)
The above is only one of a number of possibilities; a war between several of the major oil producing states could have the same end result. Any major world war would result in interruption to the global economy and possibly make it impossible for us to maintain our industries; which have become, I suspect, totally dependent upon a free flow of replacement parts from overseas. Another possible trigger for a cascading collapse in our technology-dependent civilization is an influenza pandemic with the severity of that of 1918.
The Western world is becoming increasingly dependent on computers. In addition we use microprocessors in many of our machines. What would hapen if replacement parts became unavailable due to terrorist attack, world war, or pandemic?
A prudent society, I suggest, should have contingency plans for such a situation. Does ours have? Or do we just hope that it doesn't happen - or can't happen?