metis
aged ecumenical anthropologist
I would have no problems launching a counter attack if there were no options. There's to many people on the world anyway. If I could swing it though, I would launch a full land invasion of Russia and try to take over the country that started the attack and use there land for our suvivors.
It depends on a lot and I have no idea what other options there are but I would not let them go without penalty.
But the vast majority of people you would be killing would be totally innocent, and what exactly would be accomplished?