Any civilization capable of sending missiles across the galaxy should be more than capable of simply sending a tight beam of gamma radiation to sterilize the planet. No need for earth shattering explosions. Just a flood of radiation engulfing the planet for a minute or two and everything not buried a mile underground will be dead. There would be no warning either.
And that's if they don't bother to just blow up the sun.
Dude... Just no. They have not thrown the kitchen sink at us. Tear gas, unconstitutional arrests, even shooting people in isolated incidents is not even fucking close to how bad it could get. Short of sending an M1 Abrams? How about just unloading full auto rifles into a crowd? How about taking out targets with predator drones? FFS, they only just started deploying LRAD.
Things can, and very well may, get so, so, SO much worse even before tanks start rolling in the streets.