im not sold myself, although i do agree a dirty bomb is much more likely than an actual warhead, its far far too easy to do
although an engineered virus is a big worry of mine, if done right it could wipe us all out before we knew it, theres this thing called quorum sensing, and its a way by which a bacterial or viral population can ensure it infects a good portion of a population before becoming active, essentially it works via chemical signals, and once a certain chemical reaches a certain concentration, then the virus or bacteria "know" to activate, so feasibly 10% of the population of a city could become infected before anyone starts showing symptons
....not to mention there really is mind control bacteria....for instance theres one called toxoplasma and its target host is a cat, so it tends to infect humans and mice (humans as a remenant from when we were hunted by big cats) and basically what it does is it makes your brain release large amounts of dopamine whenever you take a risk, like crossing a road dangerously, or going somewhere you could get attacked by a predator, so in essense this parasite will make you very rash and brazen and affecting your behavior, with some designed tweaks to such a virus it wouldnt be a giant leap to make it inactive until it reaches a certain population concentration and also change how it modifies your behavior, like making you very very very angry (somewhat like 28 days later).....now most of this sounds very unlikely, and it would be hard to do, it is possible, and for me thats a very very scary threat.
but yes the yanks will almost undoubtedly cause the next war, and they'll be proud of it, like school children breaking someones science project.