As Ukrainian forces push to liberate occupied tracts of their country, there will be a strong temptation for them to attack transportation, storage and staging facilities inside Russia itself. Since the Russians have been shelling indiscriminately deep into Ukraine, it is going to be very difficult to dissuade Ukrainian commanders from moving their artillery systems right up the border.

Meanwhile, I suspect that Putin's armed forces are desperate for any excuse that will justify their escalating the war, perhaps even convincing themselves that the NATO hordes are pouring into Mother Russia. The threat of WMDs is now magnified.