Neutron (Quantum 6)

[Phantasm]

So, the other usual suspects:

The Cabal probably wouldn't resort to advanced technological means such as a neutron bomb unless it was part of a scheme to raise the mana level via the instant deaths of millions. Presumably there is no evidence to indicate a rise in mana; furthermore I can't think of any magical artifact they'd need from there that couldn't just be stolen from a museum or tomb.

Reich-5 is probably the best contender, assuming the Chronobahn crosses quantum levels (not sure if it does or not); they'd use neutron bombs, but the lack of the rise of the National Socialist party in the German states seems to indicate that if they had a hand in it they're not gaining the foothold they want. They are also not moving in to take over in the chaos, so that's another strike against them being the culprit.