Well, in The Moon is a Harsh Mistress, they had a bit of a revolution on the moon and started throwing rocks. Having the power generators situated on earth would keep that scenario at bay.
Still, a good space navy & sufficient sensor coverage is the only real mitigation against people playing with KEWs.
Still I wonder how dangerous a mass driver launched rock from the Moon is as long as the mass driver is originally configured for cargo launch, so "just" a few km/s.
Still could do quite a bit of damage if it hits something but if it does not do any.manuevers &could be detected, then there should be ~days for arranging something to collide with it and turn it to plasma or dust.
But I guess if they could spam it & be creative with the resulting trajectories. Still the mass driver is fixed on the Moon surface, so it might not survive for long until it is itself hit by something in retaliation.
Even having ~days would be pretty impractical if it turns out they target a major metropolis. Let's say they target NYC, which had on the order of 18 million inhabitants in 2022. Moving 18 million people in a few days is going to be a major undertaking that few governments would be able to manage without significant prep time. Just imagine the traffic jams you get, exacerbated by cars stopping in the middle of the highway without fuel because the current fuel infrastructure is not going to manage the demand of several million cars passing by. Apart from the amount of people killed, the property damage would be insane.
IIRC the early Cold War civil defence plans (pre ICBM) envisioned just that - quickly truck/bus people out from urban centers to dispersal locations when nuclear bomber stream has been detected. It apparently worked to some degree and was regularly rehearsed in some areas. Then ICBMs with <30 minute warning windows kinda made it moot.
